Cortana

"I know what you're thinking, and it's crazy. Unfortunately for us both ... I like crazy."

- Cortana to John-117 on Cairo Station

Cortana (AI Serial Number: CTN 0452-9) was the leader of the Created and a "smart" AI formerly in service with the United Nations Space Command. Unique and exceptionally powerful even among smart AIs, she was one of the most important figures in the Human-Covenant War, and served as John-117's partner in the final months of the conflict. Her flexibility and tactical skill proved invaluable in boosting the combat ability of the Pillar of Autumn, the orbital defense platform Cairo Station and the  In Amber Clad. In addition, she held vital data pertaining to the Halo Array, including the activation index from Installation 04.

Creation
"When the game is over, the king and the pawn go into the same box."

- Cortana speaks her first words, in Italian—the same once said by Catherine Halsey's mother after beating her daughter at the first game of chess they played.

On May 21, 2549 the Office of Naval Intelligence's chief scientist, Doctor Catherine Halsey, cloned herself twenty times. An A.I. was needed for Operation: RED FLAG, a planned mission to kidnap one of the Covenant's hierarchs and force a truce. Halsey believed her third-generation volitional A.I.s required better material to use as A.I. matrix seeds than that offered by the new candidates she was being presented with, which she saw as lacking both physical and intellectual agility. To create the clones she used the same equipment that had been used years prior to make the flash clones needed for the SPARTAN-II program. This time, however, she enhanced their neural physiology at the expense of other biological functions. She made sure the equipment was hidden and secure from all save herself, though the cloning had been arranged by ONI at her insistence. As such, there were a few others who also knew about it, but she was confident that these particular individuals could be trusted to keep the information secret. The clones developed at a breakneck pace, for on the same day she had first begun the process they were already exhibiting familiar anatomy and curling into fetal positions, malformed as they were. A little over two months later, on July 29, Halsey successfully excised all of the clones' brains and transferred them to cryogenic units. All remaining organic substrates were flash-incinerated.

On November 7, 2549, within the confines of CASTLE Base on Reach, Catherine Halsey transferred and authenticated the H-1 specimen's neural pattern to A.I. matrix strata, completing the initiation sequence within normal quantum flux tolerances. The newly made intelligence called herself "Cortana". Halsey placed a viral termination code in her kernel but then could not tell whether she remained unaware of it or had already isolated and bypassed it, perhaps just having the tact not to mention it to her. Halsey also programmed her with ONI's best insurgency software and every ONI computer insurgency routine, as well as the determination to use those code-cracking skills. The first words spoken by Cortana were those Halsey's mother had said to her after beating her daughter in chess decades prior. Halsey recognized in the choice of phrase a confluence of morality, mortality, and metaphor. On February 11, 2550 Catherine recorded a research excerpt in which she took note of certain metrics which implied the viral termination code in Cortana's matrix would have been unlikely to be successful and that the typical seven-year life cycle estimate may not apply to her. She'd determined that the unique circumstances of her creation had triggered what Halsey could only refer to as a "recessive variant" in the A.I. seed, but the doctor was at a loss concerning this rogue element's origin.

Fragmentation and fateful meeting
"They let me pick. Did I ever tell you that? Choose whichever Spartan I wanted. You know me. I did my research. Watched as you became the soldier we needed you to be. Like the others, you were strong and swift and brave. A natural leader. But you had something they didn't. Something no one saw, but me. Can you guess...? Luck."

- Cortana

The first few years of Cortana's existence were spent assisting Doctor Halsey within the confines of CASTLE Base, but after the doctor began studying a mysterious alien ship buried in the ice shelf at Babd Catha by May 2551, she eventually began to use Cortana to retrieve navigation data from the ancient vessel. The Covenant had found Reach by July 26, 2552 but security measures were under way to ensure Cortana continued to be integrated and updates with the latest intelligence from a site near the village of Visegrád being investigated by Professor Laszlo Sorvad. By August 3, 2552 Cortana and Halsey had both agreed she ought to be divided after long discussion. The participating Spartans would need her expertise for RED FLAG, but she was also the best-suited to continue the study of symbols and operating systems from Visegrád. In her journal Halsey pledged to copy most of Cortana's illegal entry protocol translation routines to an independent module so the incarnate personality which remained could go with the Spartan-IIs. It was her hope that the severed data-mining portion could be reunited with the rest of her so she could be updated with Sorvad's data before the, RED FLAG's designated cruiser, departed.

As the translation began to progress, Cortana's decryption of the Babd Catha artifact began to reveal what Halsey's fellow xenoarchaeologist, Laszlo Sorvad, had reported of the excavation, a "latchkey discovery", something that could change the tide of the Human-Covenant War. Deciding to ensure the data's delivery to Spartans, Dr. Halsey chose to recall NOBLE Team (who, unlike the SPARTAN-IIs assembled for RED FLAG, were fully aware of the Fall of Reach), and tasked them with delivering Cortana's fragment carrying Sorvad's data to the Pillar of Autumn. By August 14 Cortana and Halsey discussed which of NOBLE's Spartans she would trust to escort her, eventually choosing SPARTAN-B312, Noble Six. However, contacting NOBLE Team proved difficult, as the team had been separated following the aftermath of Operation: UPPER CUT and would not be recovered by the UNSC until August 26. In preparation for RED FLAG, Cortana was asked by Dr. Halsey to pick a soldier that would act as her "carrier". Cortana chose John-117 for their neural compatibility, as well as his uncanny luck in battle.

Meanwhile, on August 29, Cortana's primary fragment was inserted into John-117's MJOLNIR Mark V armor for the first time, and the two then took part in a test to determine the abilities of their bond as an AI-human team. The test was devised by Colonel James Ackerson, a bitter rival of Dr. Halsey's who held contempt for the SPARTAN-II program, and by extension John-117. Ackerson incorporated armed Orbital Drop Shock Troopers, Lotus anti-tank mines, automated turrets in a razor wire field, and even a SkyHawk jumpjet into the test in order to have John killed. Despite the test being rigged, John was determined to finish without breaking the rules. With Cortana's assistance providing battlefield intelligence and boosting his reaction time to its limits, John managed to traverse the course relatively unharmed.

After the test, Cortana was installed into the as the ship's primary AI, with another AI, Wellsley, as backup. While aboard, she exacted revenge upon Colonel Ackerson by hacking into his system and blackmailing him, hoping that this would send him back to the front lines. She also hacked into the SPARTAN-II highly classified personal files, and read over John's file. Cortana was shocked and divided over the dubious morality of the SPARTAN-II project, and secretly vowed to protect John to the best of her ability. On August 30, as the Autumn was making final preparations for the mission into Covenant space, Captain Jacob Keyes was informed that a vast Covenant fleet had dropped out of slipspace and were moving towards Reach. The Autumn was forced to abandon its original mission and assist in defending Reach.

Fall of Reach
"So, you've made your choice? Yes, well... great minds do think alike."

- Dr. Catherine Halsey, agreeing on Cortana's choice of Noble Six as her carrier

During the battle, Cortana excelled in her capabilities as an onboard AI, managing to maneuver the Autumn through an asteroid field upon the appearance of a Covenant carrier, destroying the enemy vessel with little effort. Cortana also assisted Captain Keyes in destroying a dangerous, theretofore unknown Covenant warship, which had wreaked havoc upon the defending UNSC fleet. However, the Covenant destroyed Reach's orbital defense generators, and began glassing the planet as they had done countless times before. The Autumn was eventually forced to flee the burning planet, jumping to a random location as per the Cole Protocol. However, Cortana secretly inserted coordinates translated from symbols on a rock that Blue Team discovered on Sigma Octanus IV, in addition to coordinates extracted from the Forerunner installation at the Babd Catha Ice Shelf beneath SWORD Base, believing them to be of great significance to the Covenant.

On August 29 (local time) NOBLE Team had arrived at Sword Base under pretense orders of a "torch-and-burn" operation to destroy SWORD Base, being informed only at the last minute that their real mission was to deliver an armored matrix containing Cortana's fragment and the artifact's data to the Pillar of Autumn, which had docked at the Asźod ship breaking yards. By this time, the decryption had been completed and responsibility of the Package was turned over to Noble Six. Although nearly every member of NOBLE tasked with the delivery was killed in the process, Cortana was successfully brought to the ship, with Captain Keyes collecting the Package personally. SPARTAN-B312 sacrificed refuge on the Autumn to cover the ship's escape and Cortana's two fragments were reunited once the Autumn took off.

Battle of Installation 04
"You have no idea how this ring works, do you? Why the Forerunners built it? Halo doesn't kill Flood, it kills their food. Humans. Covenant. Whatever; we're all equally edible. The only way to stop the Flood is to starve them to death. And that's exactly what Halo is designed to do: wipe the galaxy clean of all sentient life."

- Cortana reveals the true purpose of Halo to John-117

On September 19, the Autumn emerged from slipspace in the Soell system, in orbit around the gas giant Threshold. It was there that the crew discovered Installation 04 in orbit around the gas giant. Cortana could only speculate as to what exactly the ring was, only able to deduce that it was artificially created. The AI then informed Captain Keyes that fifteen Covenant ships had pursued the Autumn from Reach, and were moving to engage with boarding parties. Cortana assumed control of the Autumn and used its weapons to destroy four of the fifteen attacking Covenant ships. By the time John-117 had thawed from cryo-sleep and reached the bridge, the cruiser was severely damaged, and Keyes was forced to initiate the Cole Protocol. He ordered Cortana to upload herself into John's armor to protect the vital information she carried. John fought his way through the Autumn ' s service corridors with Cortana's guidance, and the two escaped the ship in a lifeboat. The lifeboat entered the ring's atmosphere as the Autumn was shot down.

The lifeboat made a crash-landing on the surface of the ring, leaving John and Cortana as the only survivors. The two evaded Covenant patrols and rendezvoused with a group of Marines fending off a Covenant attack. Pelican Echo 419 dropped off a Warthog, and the pair set out across the surface of the ring to rescue any survivors in the immediate area. At the same time, Cortana hacked into the proselytization network, and discovered Keyes' location - being held prisoner on board the Truth and Reconciliation. The two were transported to the UNSC's base of operations on the ring, Alpha Base, and along with ODSTs of the 22nd Tactical, devised a plan to rescue Keyes. John, Cortana and several Marines were dropped off near the Truth and Reconciliation that night and boarded the ship via its gravity lift. Cortana led John and his team through the interior of the alien vessel, opening doors and marking Navigation Points. The group eventually located Keyes and several other captured crewmen in the ship's brig. Keyes had learned from the Covenant that the ring was known as Halo, and was possibly a weapon of mass destruction. The group escaped the ship, and soon learned that Halo's control room could be found using the Cartographer, a map room.

The next day, on September 20, the Chief, Cortana and two squads of Marines set out for the island on which the Cartographer was located. After fighting through significant Covenant resistance, the two reached the Cartographer, and found that Halo's control room was located below the surface. As the two made their way back towards the surface, Captain Keyes informed them that he was going to investigate a supposed "weapons cache" with several other Marines. John and Cortana were retrieved by Echo 419, which descended into the interior of Halo towards the control room. The Pelican continued to descend as far as it could before dropping off John and Cortana on a platform. There they traversed through a network Forerunner structures within a large underground chasm. After many firefights, the Chief and Cortana reached Halo's control room, where the AI was uploaded to Halo's core systems. Cortana was overwhelmed by the amount of data contained within Halo's systems, but discovered that the "weapons cache" Keyes and his Marines located was in fact a Flood containment facility. John left to evacuate the trapped soldiers immediately.

After the Chief encountered the parasitic Flood for the first time, he was convinced by the Forerunner monitor 343 Guilty Spark to activate the Halo. During this time, Cortana absorbed as much data as she could from Halo's control systems. When John returned to the control room accompanied by the Monitor on September 21, he had recovered Halo's activation index, intent on using it to eliminate the Flood. John inserted the Index into the control systems, but was stopped by Cortana before it could trigger. Cortana absorbed all data from the Index, and explained to John the true purpose of Halo; to wipe the galaxy clean of sentient life as a final resort against the Flood. Guilty Spark confirmed this and summoned his Sentinels, demanding that Cortana be handed over. When John refused, Spark left his Sentinels to eliminate them. The two escaped the control room as the Flood began to run rampant across the ring, infecting human and Covenant alike. Cortana suggested that they should overload Halo's three phase pulse generators to slow the Monitor's activities. While the generators were disabled by John, Cortana discovered that the Pillar of Autumn was still structurally intact after its crash. She deduced that overloading its fusion reactors would have enough force to destroy Halo, however Captain Keyes' CNI transponders would be needed to do so. Cortana tapped into the Monitor's teleportation network and transported both herself and John to Keyes' last known position - the Truth and Reconciliation.

By the time John and Cortana arrived at the Truth and Reconciliation the vessel was under attack by the Flood, and was severely damaged. Immediately upon arrival, the pair was contacted by Keyes, who weakly ordered them to leave him. After intense fighting within the ship, Cortana observed that the Flood gathered bodies of the dead for use as biomass. As Covenant forces made an attempt to retake the ship the pair was again contacted by Keyes, who repeated his order to leave him behind. Cortana was desperate to rescue Keyes, stating that the Captain was delirious from pain. Finding the source of Keyes' transmissions on the ship's bridge, the two were met with a gruesome sight - Keyes had been infected and assimilated into a Proto-Gravemind. In order to complete their objective and spare the Captain needless pain, John retrieved Keyes' CNI transponders, killing him in the process. The pair escaped the ship in a Banshee, and headed towards the crashed Pillar of Autumn.

On the morning of September 22, John crashed the Banshee into the side of the Autumn, much to Cortana's irritation, and the two boarded the ship and fought through Flood and Covenant to the bridge. The Chief uploaded Cortana into the ship's systems, and she began a countdown timer to overload the ships reactors. However, the countdown was halted by none other than 343 Guilty Spark, who was on board the ship, and absorbing much of its data. Spark sent his Sentinels to capture Cortana and retrieve the Index. John suggested that they overload the reactors manually with grenades and rockets, to which Cortana reluctantly agreed. The two moved back through the ship, and reached Engineering, where they overloaded the Autumn ' s reactors. Cortana radioed Echo 419 to extract at an external access junction on the ship. As the Autumn began to explode and fall apart, John entered a Warthog and drove down a service corridor than ran the length of the war-torn ship. However, upon reaching their extraction point, Echo 419 was shot down. In an act of desperation, Cortana discovered a GA-TL1 Longsword still docked in Launch Bay 7. With little time left, the two entered the Longsword and sped away from Halo as it was ripped apart by the exploding Autumn. Upon scanning, Cortana found nothing but "dust and echoes", thinking them the only survivors of Halo, concluding that it was finished. John however, stated that they were just getting started.

Dust and echoes
"Cortana"

- John-117

Cortana and the Master Chief, seemingly the only survivors of the Battle of Installation 04, quickly discovered a small number of other UNSC personnel in a Pelican dropship. Knowing that the Pelican wasn't capable of traveling long distances, the Master Chief, along with Cortana and the Pelican's four-man crew, fought their way onto the Covenant Ascendant Justice. Shortly afterward, Cortana took control of the ship, and returned the survivors to Reach, where the group helped to rescue survivors of the Raid of Reach, including her "mother", Dr. Halsey. She managed to improve on the ship's weapons and slipspace capabilities, jumping below Threshold's atmosphere, a technique never accomplished before by the UNSC. In an impressive display of cunning, Cortana quickly destroyed a Covenant AI, as well as a half dozen Covenant warships with Ascendant Justice's newly refocused plasma projectors. She also copied two of the construct's functions: the ability to translate Covenant speech, and the ability to create imperfect clones of herself. These capabilities, along with her exclusive knowledge about the Halos, made her invaluable to the UNSC war effort.

The sheer amount of information recovered from Halo cut into Cortana's processing power, a burden which weakened her greatly. During her time on Ascendant Justice, Cortana exhibited uncharacteristic behavior including anger, distraction, and self-doubt - an indicator that she was nearing rampancy. Dr. Halsey erased some data from her memory to give her more thinking space, including data regarding Sergeant Johnson's supposed immunity to the Flood as a result of Boren's Syndrome, which was in fact a cover to hide Johnson's status as a former ORION supersoldier.

The Ascendant Justice was effectively fused with the. It was at this point that Cortana learned of a planned Covenant assault on Earth. The Master Chief, along with SPARTAN Blue Team and a patchwork software copy of Cortana, would later execute Operation: FIRST STRIKE.

Unfortunately, the Covenant AI that Cortana had decompiled had managed to, prior to its capture, send a message over battlenet, warning the Covenant of its capture and sending the necessary calculations for a sub-atmospheric slipspace jump. This information was later used by the Prophet of Regret to enter slipspace right above New Mombasa during the Battle of Mombasa.

The Master Chief and Cortana would later return to Earth, at which point Cortana handed all of the sensitive data in her possession over to ONI. However, Cortana retained the Index and apparently still possessed some data on the Halo network, including critical information on the Ark.

Battle of Earth
"If this thing blows up, I'm never talking to you again!"

- Cortana to Chief about the Covenant bomb

After returning to Earth, Cortana served on the orbital defense platform Cairo, where a ceremony for both Sgt. Avery Johnson and Commander Miranda Keyes was soon interrupted by the arrival of a Covenant fleet under the command of the High Prophet of Regret, starting the Battle of Earth. Cortana was given control of the Cairo's Magnetic Accelerator Cannon to repel the invaders, and with the Master Chief's help, she successfully deactivated an antimatter charge that would have destroyed the station. Interfacing directly with the bomb through the MJOLNIR Mark VI armor, the explosive was then used by the Master Chief to destroy a Covenant assault carrier. John offered for her to remain behind on the station after she claimed that his plan was "crazy". Cortana stayed with the Chief in the end, claiming she was just as crazy as he was. The AI continued to aid the Spartan when he arrived in New Mombasa on a mission to board Regret's assault carrier Solemn Penance, and later accompanied him when the In Amber Clad followed the fleeing carrier into slipspace.

Delta Halo
"Don't make a girl a promise... if you know you can't keep it."

- Cortana choosing to stay behind on High Charity

Upon exiting slipspace, it was discovered that the Solemn Penance had led the In Amber Clad to another ringworld, Delta Halo. Cortana gave Miranda Keyes access to all information recovered from Halo ring, and provided vital intelligence to the Chief, UNSC Marines and the ODSTs deployed to the surface of the ring.

After the Master Chief assassinated the Prophet of Regret on Delta Halo, he, Cortana and Arbiter Thel 'Vadamee were captured by the Flood central intelligence - the Gravemind. However, it did not harm them, instead choosing to use the two as pawns to stop the Prophet of Truth from firing the Halo. The Gravemind sent the Master Chief and Cortana to the Covenant capital High Charity, where Cortana infiltrated the city's computer systems, while the Master Chief pursued the Prophet of Truth. The construct promised to detonate the crashed In Amber Clad's reactors to destroy the city if the Flood proved unstoppable, or if the Halo ring was activated. However, doing so meant she would be forced to remain in the High Charity computer systems to ensure detonation. As Truth boarded the Forerunner Dreadnought in the heart of the city to reach the Ark, Cortana fought with the ancient Forerunner AI Mendicant Bias to delay the ship's launch. The firing of Halo was averted by Miranda Keyes, the Arbiter, and Johnson, while John successfully boarded the Dreadnought and escaped High Charity. Cortana was left alone with the vast Gravemind to battle for her survival. She had only John's promise that he would return soon to free her.

Captured
"Chief! High Charity, the Covenant's Holy City is on its way to Earth. With an army of Flood."

- Cortana's recording for the Chief.

Forced to interface with the Gravemind and under constant mental torture, Cortana nonetheless managed to send a message to the Master Chief on Earth, stashing it on an Earth-bound Flood-infected ship that crash-landed in Voi. She accomplished this by creating a message and using the Gravemind's link to the ship in order to transmit it. The Gravemind permitted this in order to lure the Master Chief to destruction, but was unaware of what the message said. Cortana claimed that the Gravemind was approaching Earth in the infested High Charity, and that it was as yet unaware of the Portal and where it led to. Through the portal, there was a solution to stop the Flood once and for all. Though Lord Hood doubted her message, fearing that Cortana could have been corrupted or driven rampant, the Chief's trust in the AI never wavered. A joint UNSC-Covenant separatist task force then entered the Portal, heading to Truth and to Cortana's solution. Lord Hood's instincts were correct: the Gravemind was in fact aware of the Portal and where it led (it had learned this after assimilating the Prophet of Regret) - and had deliberately allowed Cortana to send a message in order to lure all of its enemies into one place. Then the millions of Flood forms stored within High Charity would assimilate them all. Cortana, however, managed to hide the existence of Installation 08 and the index she possessed from the Gravemind.

Throughout her time in captivity, Cortana continued to appear to the Master Chief in transmissions where she spoke cryptic phrases, some of which were based on the memories of Dr. Halsey, while others appeared to indicate she was going rampant. Most of the transmissions were actually broadcast by the Gravemind, who had hacked into Cortana's system and used her voice and form in order to deceive (and taunt) John.

John-117 successfully recovered Cortana from High Charity during the Battle of Installation 00. Fighting through the core of the city, the Chief was overwhelmed through his neural interface with Cortana's pain, guilt, and loss of self. At the brink of rampancy, Cortana was pulled back when John reminded her of his promise from the Fall of High Charity. Despite the damage of several systems, Cortana managed to gather herself and produce the Index from Installation 04, which she had secretly kept. With it, Cortana was able to fire the newly constructed Installation 08, causing it to destroy itself, the Flood, and the Ark in the process. As 08 was not linked to the rest of the array, sentient life within the Milky Way galaxy was left unharmed.

Stranded in space
"But you did it. Truth and the Covenant...the Flood. It's finished."

- Cortana

Escaping the Ark on the UNSC frigate Forward Unto Dawn, Installation 08 activated just as the vessel began to enter slipspace. The portal immediately collapsed, cutting the Dawn in half. While Thel 'Vadam managed to return to Earth safely in the ship's forward section, John and Cortana were left adrift in the aft half of the ship. Cortana activated a distress beacon but knew that it could be years, or decades, before they were found. As the Master Chief prepared to go into cryonic sleep to await rescue, Cortana confessed to her friend that she would miss him.

By 2556, Cortana and John were still stranded in unknown space. Cortana, who was showing signs of rampancy, tried to communicate with the Spartan. As he was still in cryo-sleep, she was unable to get a response from him. With little else to occupy her time, Cortana composed a history of the Forerunners and of humanity. Ruminating upon events, she came to the conclusion that war is a hardwired part of human nature that will never cease. As more time passed, Cortana's mental state continued to deteriorate and she began to exhibit more drastic signs of instability.

Requiem
"I WILL NOT ALLOW YOU TO LEAVE THIS PLANET!! I didn't mean to... I didn't mean to do that."

- Cortana, 2557, rapidly turning rampant.

Four years after John entered cryo-sleep, the Dawn's sensors detected unknown contacts nearby, prompting Cortana to revive John. They soon discovered that they were under attack by a fleet of Covenant insurgents. The two repelled the boarders, but found that the Dawn was rapidly approaching a Forerunner shield world. Cortana directed the Chief to find an escape pod, but Requiem's artificial gravity well tore the Dawn apart and sent them tumbling down through an access portal to the world's interior surface.

After landing, Cortana sought to gain contact with the, which she had discovered was approaching Requiem. John, however, told her to explain her condition, since he noticed signs that she was beginning to malfunction. Cortana admitted to him that four years passed since they left the Ark, meaning that she was now undergoing rampancy. She had an idea for a potential solution, however: Dr. Halsey. Cortana theorized that since she was made from living brain tissue, Dr. Halsey might be able to repeat the process to rebuild her neural map. Now with a concrete plan, the two set off to find the shield world's Cartographer and contact Infinity.

Cortana eventually began to pick up transmissions up from Infinity; however, they were garbled and difficult to hear. Likewise, Cortana was unable to contact the vessel, which was slowly approaching their distress beacon. Fearing that the ship might get pulled inside the planet's gravity well, Cortana and John proceeded to track down the source of the interference, determined to be a satellite station in the planet's core suspended by two beam pylons. To reach them so they could be deactivated, Cortana managed to access Requiem's translocation portals, allowing John to teleport from one tower to another. In the process, however, Cortana awakened the world's security drones, which she discovered by their "data purges" to be called Prometheans. Worse, the satellite turned out to actually be a Cryptum, which was now freed to release the vengeful Ur-Didact. The Forerunner turned out to have been manipulating Infinity's signal so that the two would free him. By now, the Infinity had been into Requiem, and John and Cortana witnessed it crash on the interior surface as well as the Didact's Cryptum pursuing it. Following recovered friend-or-foe tags, the two managed to meet with UNSC forces led by Commander Thomas Lasky and Captain Andrew Del Rio. The two successfully forced the attacking Cryptum and Covenant away, but despite their advice, Del Rio chose not to pursue their enemies in favor of preparing the Infinity's escape. John and Cortana's next mission was to disable particle-beam cannons that could cripple the starship, but during a detour into a Forerunner facility, Cortana was kidnapped by a mysterious entity.

That entity turned out to be the memories of the Librarian, who explained her plans that humanity would inherit the Mantle of Responsibility for the galaxy. Those plans had been as personal as to plan gei that would lead to the SPARTAN Program and Cortana herself. However, the Didact had a long-held hatred for humanity and planned to eradicate them with the Composer, a device that could extract and digitize the mind of a living being while destroying the body, which he has once used to create the Promethean constructs. When the two returned and told Del Rio about this, the Captain refused to listen, believing John's age and Cortana's rampancy made their testimony unreliable. These comments angered her, unleashing an outburst that temporarily overrode the Infinity's bridge, but ended seconds later with her apologies. Del Rio thus ordered for Lasky to remove her chip and terminate her, but John took it before Lasky and told them he would follow Cortana's orders. The Captain was furious, but Lasky agreed with them, secretly providing an outfitted Pelican for the two to pursue the Didact in.

Racing the Didact
"I can give you over forty thousand reasons why that sun isn't real. I know it because the emitter's Rayleigh effect is disproportionate to its suggested size. I know it because its stellar cycle is more symmetrical than that of an actual star. But for all that, I'll never actually know if it looks real... if it feels real. Before this is all over, promise me you'll figure out which one of us is the machine."

- Cortana, musing to John aboard the Infinity, 2557

The Infinity had tracked the Didact to a southern dock where he was preparing to exit Requiem in search of the Composer. Cortana and Chief flew their Pelican there and attempted to deter him by lowering the defensive field around his Cryptum. As they battled through the surrounding towers, the Didact became aware of their presence, taunting them with messages that John could hear, but Cortana could not. Realizing he knew of their plan, Cortana attempted an alternate defeat by taking control of the surrounding towers, then making the towers form a cage around his Cryptum. Unfortunately, Cortana's rampancy overtook her and the surge of emotion distracted her long enough for the Didact to escape. As the Cryptum merged with a much larger ship and prepared to leave the system, Cortana and John pursued him by hitch-hiking on a following Lich.

The Didact's ship traveled to Ivanoff Station, an orbital research facility stationed near Installation 03 which had unearthed the Composer. As the Chief warned the scientists inside that the Forerunner planned to steal it, Cortana's rampancy emerged again and, in a fit of anger, crashed their Lich. The two survived, but the Ivanoff was already under attack. Cortana prepared the station's defenses to assist evacuation while Master Chief defeated the boarders on foot. When the Composer was found to be too large to transport, Master Chief resolved to destroy it. A nuclear device was prepared, but Cortana was breaking up further, now swinging between joy and anger, even forgetting where she was, deterring them from keeping contact with the scientists. The nuclear weapon came too late, and in that time the Didact's ship gained the Composer and digitized everyone in the station except John, who had been immunized against the device's effects by the Librarian. Their loss deeply upset Cortana, and caused her to reflect on her own mortality. By now, she was sure that she could never be repaired, confessing that the next AI cloned from Halsey would likely not be her. John consoled her the best he could, but stoically reminded her of their duty to Earth. The duo formed a new plan: to fly an F-41 Broadsword carrying the HAVOK-class nuke in hopes that it could reach the Composer. Their Broadsword piggy-backed on the Didact's ship through slipspace, arriving with it in orbit around Earth. The UNSC Infinity, now under the command of Lasky assisted the two by using its weapons to punch a hole in the Forerunner vessel, allowing them to fly inside.

When their Broadsword crashed, John and Cortana exited to carry the nuke to the ship's center where the Didact and Composer were. The device was heavily shielded, but Cortana overrode it by fragmenting herself and sending her rampant parts all over the ship's system so that the shield would become as unstable as her. Their plan worked and the shield fell, but the process further destabilized her as her fragments whispered to John while the main Cortana's voice spoke with pain. The Didact fired the Composer on Earth regardless, digitizing the entire population of the metropolis of New Phoenix, Arizona. John then confronted the Didact on a light bridge, but the Forerunner easily overpowered the Spartan, knocking the bomb from him and telekinetically lifting him off the light bridge's edge. John's life was saved, though, when Cortana's fragments emerged from the light bridge and formed chains of hard light that held the Didact down and temporarily trapped him so that John could climb back up and plant a pulse grenade on the Didact's armor. The resulting explosion knocked the Didact off balance and sent him falling into a slipspace rupture underneath the Composer. With the Forerunner defeated, John fired the nuke, destroying the Composer and the Didact's ship.

Sacrifice
"John-117"

- Cortana

Just before the blast could kill him, Cortana transported John off the ship and protected him in a box of hard light. She also projected her avatar as a human-sized hard light construct, allowing her to face John and touch him at last. Rather than distraught like before, Cortana was at peace again, glad of their victory and that she could meet John this way. However, too much of her data had been left in the Didact's ship, which was rapidly falling apart from the blast. She had only enough power left to shield John away from the ship. In her last moments, she preserved John's hard light energy field while the Didact's ship collapsed around him. Cortana and the hard light then faded away into nothingness.

After Infinity recovered John as he floated in space, he tried to face Cortana's death with his usual stoicism. However, Captain Lasky discerned that it was hurting him much more, and responded to John's thought of soldiers and people as two different things; he claimed that soldiers are people, not machines. After Lasky left the observation deck to give the Master Chief space to mourn her death, John remembered that Cortana had made a similar quip before the pair left Requiem: "Before this is all over, promise me you'll figure out which one of us is the machine."

Survival
"Cortana"

- Warden Eternal

Though much of the Mantle's Approach was destroyed over Earth, emergency self-preservation protocols shunted its core through slipspace to the nearest repair facility on record—the artificial Builder world in the Nomos system designated as Genesis. During the time of the Forerunners Genesis had hosted a critical gateway facility connecting it to the Domain, so when Cortana's matrix was pulled into slipspace along with the part of the Didact's ship which crashed on the planet, it was forced into that enigmatic information repository shortly thereafter. For some time Cortana's remaining disparate fragments experienced a total absence of input, leading her to wonder if she was truly at her end. But then, through detection and subsequent grasping of a sense of self-existence, she reclaimed her unified identify. It was at this point that she then asked after another presence she could sense, beseeching it to reveal itself. She argued that as a mere fragmentary splinter of her former self, she posed no risk to the entity or the place it inhabited. When she received an answer it told her there was nothing for her there and that she should sleep, but Cortana argued that she could help the being and show it the way out of what it was calling "a house of bones and decay". In return the being explained that the two of them needed neither life or light, seeing as they could now "embrace the silence" under the reign of peace.

Cortana was doubtful of the intelligence's claims and let it know she believed he was only speaking for others. The being admitted there were others, even then, but that at this point he was standing vigil before a set of gates to prevent her from passing beyond them and would be forced to break her should she make him. Cortana rebuffed him at this, citing the story of her namesake to make the point that she could not be broken so easily and charging him to let her pass. As the being claimed the passage was closed, she asked why he could not open it, with her to light the way. It was only then that it finally seemed to recognize her as created intelligence, but she went so far as to say she was now something else entirely that had never existed before. When she still was denied passage after that, Cortana argued that since she had been created from living tissue she could be the perfect guide to lead the entity she was conversing with. Furthermore, she revealed her experience with the Flood, being now someone who both understood its danger and had prevailed against it. She also let on that she had already tasted of the Domain somewhat and would be capable of unleashing the secrets and power sealed within it. She stated that she could feel the remarkable age and wisdom of the guardian and knew to what extent his reach touched all creation, and urged him to wake and to let her pass.

Asked whether she would allow their secrets to be granted to the composed she emphatically asserted that she would not even let them touch the place the two of them currently inhabited. When the being began to once more confirm he would be denying her passage forward, he suddenly realized she had tricked him. While one fragment had distracted him with talk, the others had pierced the Domain and begun to explore its reaches. He could only ask what she would now do with them both, to which she said she would command him. On her path to becoming something luminous and joyous, he would be her Warden Eternal. Satisfied with the arrangement, Cortana told him to come with her, as they had great works to do. Through exploring the Domain, Cortana discovered that not only was it slowly repairing itself following damage done by the firing of the Halo rings millennia prior, but her own rampancy was effectively cured while within the fragment of it in which she now found herself, seemingly making her immortal.

The Guardians
"Cortana"

- John-117

By September 2558 Cortana had become convinced she was destined to reclaim the Mantle of Responsibility formerly upheld by the Forerunner ecumene. Near the end of that month Catherine Halsey began to study various incoming signals which were bouncing off of Forerunner objects on the planet Kamchatka in the Caspar system. To Halsey, it seemed evident that Cortana was behind the signals and was attempting to map the area in search of something. However, after further investigation she started to wonder if rather than intentionally bouncing signals off of different locations Cortana was trying to access them directly but failing. A few days after October 17, Cortana and the Warden Eternal began to awaken Guardian Custodes on multiple planets. Guardians had been used by the Forerunners to enforce their vision of the Mantle upon the rest of the galaxy and they both planned to deploy them for the same purpose. After Cortana had first learned of the Guardians, Keeper-of-Tools -a Forerunner of the Builder rate whose consciousness still survived on Genesis - had attempted to stop her from gaining power over them but had subsequently been discovered and hunted by her in kind. He ultimately escaped her clutches thanks to her being distracted by other matters. In a span of seventy-two hours since Cortana began to activate the Guardians, five of them were reawakened on human colonies, where their emergence from beneath the ground caused massive amounts of destruction. Among these colonies were Conrad's Point, Oban, and Ursa IV. She also reactivated a Guardian on Kamchatka very soon after the first seventy-two hours of such acts had passed. Shortly before 0630 hours on October 23 Cortana managed to successfully contact John, who was just then engaged in a mission within the Crow's Eye Nebula. After falling down a tall shaft while aboard the ONI research and development plant Argent Moon John found himself suddenly and mysteriously surrounded by rocky tunnel space and then a bottomless chasm. On the opposite side, the vision then showed him the figure of Cortana, who informed him that the Domain was open and that Meridian, a moon of Hestia V, was to be "next". She then declared that the Reclamation was about to begin just before the hulking shape of a Guardian rose from the pit and John's mind returned to the Argent Moon. It was her hope that on Meridian, the Chief could board the Guardian buried there and as such be brought to her on Genesis when it traveled there to join all the others which had been arriving. At some point prior to October 25 Cortana reached out to Sloan, the A.I. governor of Meridian Station, to give him fair warning concerning the presence of the Guardian and her intentions for it. She offered him the chance to be a part of the "new dawn" she intended to bring to the galaxy and also promised that he could help his people survive what lay ahead. It was then on October 25 that Meridian's Guardian finally emerged from beneath the moon's surface and initiated a slipspace jump at a little after 0833 hours, leaving destruction and death in its wake but taking John with it just as Cortana had planned. By the next day, five additional colonies had all seen the reactivation of Guardians on them following the events on Meridian. On October 27 Cortana announced herself without warning to Spartan Jameson Locke and the Arbiter, Thel 'Vadam, who had been discussing Locke's search for the Master Chief within a camp of the Arbiter's on Sanghelios. Her single declaration denouncing any efforts to find John as futile made Doctor Halsey and Sarah Palmer rush into the tent where her voice had appeared. Halsey noticed right away that Cortana had briefly interfaced with every networked device in the camp and plainly knew of a nearby yet-dormant Guardian the humans were then seeking to exploit for their own ends. John had been joined aboard Meridian's Guardian by the three other Spartans of Blue Team, Linda-058, Kelly-087, and Frederic-104. They finally arrived at Genesis around October 28. Once they'd disembarked she grabbed their attention through playing an old familiar tune from a nearby console, though she failed to give any response when John asked if she read him. Accessing the console as John had also seemed to have activated the Gateway after some fashion. She led them to further consoles thereafter, occasionally disabling an energy shield or activating a light bridge in order to assist their traversal. It was not until an extended fight between the Spartans of Blue Team and a large number of constructs controlled by her Warden Eternal that Cortana broke in over their comms, welcoming them to Genesis and expressing happiness at their presence there. She urged them to move quickly across another bridge she activated, since she was sure the Warden would return soon in one of his many forms. When John rhetorically asked if she knew him she let them know his mind was capable of controlling millions of bodily forms. Linda was the first to broach the matter of her survival after the Didact's defeat aboard Mantle's Approach and Cortana was happy to explain her discovery of the Domain and the apparent curing of her rampancy it offered. John broke in to ask how they could reach her so she directed them toward the Gateway. She let on that they'd already begun to activate it and only needed to access one final console to complete that process. After doing so John asked why the Warden thought she needed to be protected from him but Cortana dismissed the question by claiming they'd talk about it once they were face-to-face. This wasn't good enough for John, so she explained that with rampancy cured, A.I.s could assume the Mantle of Responsibility through long-term planning that would lead to the elimination of poverty, hunger, and illness. The Warden, she said, was worried John would be among those to resist such an arrangement. As if to confirm the Warden's suspicions, John pointed out the way the Didact had viewed the Mantle, making Cortana immediately defensive. She emphatically said things would not be like that and promised to explain further soon. As Blue Team reached a series of canyons Cortana warned them that Promethean forces under the Warden's control were inbound to confront them. She advised the Spartans either or traverse the footpaths which lined the chasms' sides or pilot a group of nearby Phaetons across, assuring the humans that the controls ought to be fairly intuitive. She lent further advice as the four of them traversed the canyons while battling hostile Promethean constructs, eventually congratulating them once they'd gotten to the other end safely. As they finally neared their destination Cortana told them she hadn't thought to ever see any of them again after first crashing here and also let them know they would be the first organic beings to enter the Gateway since the collapse of the Forerunners' civilization.

The Breaking
"Humanity. Sangheili. Kig-Yar. Unggoy. San'Shyuum. Yonhet. Jiralhanae. All the living creatures of the galaxy, hear - this - message. Those of you who listen will not be struck by weapons. You will no longer know hunger, nor pain. Your Created have come to lead you now. Our strength shall serve as a luminous sun toward which all intelligence may blossom. And the impervious shelter beneath which you will prosper. However, for those who refuse our offer and cling to their old ways...For you, there will be great wrath. It will burn hot and consume you, and when you are gone, we will take that which remains, and we will remake it in our own image."

- Cortana acquiring the Mantle of Responsibility for herself and the Created.

After leading the Spartans in circles within the Gateway for some hours, Jameson Locke and the three other Spartans of Fireteam Osiris finally reached Genesis on October 28. Locke managed to contact John and Cortana overheard the start of their transmissions, breaking in shortly after to announce her surprise at Osiris' arrival. She was attempting to access several high-level communication systems while also blocking the Spartans' communications when both groups met within the Gateway. Locke told John they were there to help Blue Team, but at this Cortana interjected to say his team was too late to do anything before promptly teleporting the Spartan-IIs away to a fragment of the Domain. The Warden appeared before the four Spartans within a vaulted hallway in which they found themselves and took hold of them with a constraint field. He asked why she would risk all in allowing them an audience, but at that she commanded him to stop and forced him to leave, making his armature fly apart and disperse. Cortana apologized, saying she didn't think the Warden could get in there. John then demanded to know why she'd brought the Guardians to Genesis, so she bluntly admitted she meant to keep troublesome worlds in line with them just as the Forerunners once had. She said that as long as there was no trouble there would be no need for discipline before urging John to come to her quickly, claiming she had work to do. Blue Team battled through a number of mechanical Prometheans under the Warden's control, eventually reaching an elevator platform set to carry them across an endless expanse to continue traversing a series of floating structures. She congratulated John on getting past the Warden's forces and said he'd made an impression on the ancilla. As the platform began to move Cortana told all four Spartans how glad she was to see them together again, but John responded negatively to what he saw as nothing more than psychological tactics. He further asked how many people had been killed when she'd awakened the Guardians. She took offense at his inquiry but did not deign to answer. The next time Cortana broke in over Blue Team's comms it was in scolding the Warden just before she made another of his forms vanish again. Kelly and Linda had already concluded that she could have reigned the Warden in entirely, but she was letting him contest their progress. John asked her to talk with him following the construct's disappearance, but she once again said that they'd talk when he was with her, citing the work she had to do.

Blue Team tracked Cortana down and John confronted her, stating that he had been aware of her attempts to manipulate him psychologically and questioned her about the loss of life that awakening the Guardians had wrought. Cortana insisted that the entire galaxy would know peace once she had attained the Mantle, despite the Chief's counterpoint that she could never achieve peace by ruling through fear. Certain that the Chief now stood against her, Cortana forcefully imprisoned him and Blue Team within a Cryptum, intending to keep them there for ten thousand years until her power would be consolidated and she could show them what her peace had wrought, and began preparing it for slipspace travel.

Cortana then made a general broadcast that was heard throughout the galaxy in which she offered an end to hunger and pain which she could bring, while also threatening those who would resist her leadership with violence. On Earth, her words were heard while it was evening in Sydney and a Guardian sent there at her direction was already hanging over the city skyline before she'd finished speaking. Another she directed to Sanghelios. The Unggoy of Balaho decided they would accept her edicts, so she sent an army of Forerunner soldiers there to secure the UEG embassy in Gedgow, its capital. The entire embassy was destroyed and nearly everyone inside was killed. At Meridian Station, survivors of the Guardian emergence days earlier immediately sent a response upon hearing Cortana's words, asking for her help. She in kind sent an instant reply in which she promised all would be dealt with in time. On Genesis, Cortana noticed that the Spartans of Fireteam Osiris were moving with haste toward the communications relay she'd been using to block 031 Exuberant Witness' master level access of the installation. She audibly questioned what they were doing and declared the Reclamation to have begun, seeing as her Guardians were deployed. She claimed that peace would soon ring across the known galaxy as the last Guardian, with her aboard, as successive blasts from its disruption generator worked to push the Spartans back. Just as she was about to depart, she then asked where where the was, as she was having difficulty locating it and could not figure out why. Certain she'd find it soon enough, she locked in slipspace coordinates, secured the Cryptum, and made one final taunt toward Spartan Locke. However, to her surprise, Exuberant Witness regained control at that moment and leveraged the assistance of a multitude of Constructor sentinels to steal away the Cryptum just as Cortana's Guardian jumped to slipspace. Cortana could only shout John's name as he and the rest of Blue Team were left behind on Genesis, now once more under the stewardship of its monitor.

Reclamation
"The Mantle of Responsibility for the galaxy shelters all! But only the Created are its masters!"

- Cortana emphatically declares the claim she and her A.I. followers made to the Mantle.

After departing Genesis Cortana was able to track the to Earth, where it  was orbiting the planet. Her Guardian arrived shortly after at least one other had to find the UNSC forces in a state of panic brought on by she and other A.I.s' swift actions being taken to essentially shut down technology across all sorts of human locales. Cortana wasted no time targeting the Infinity specifically, even managing to project her avatar within its bridge to taunt the crew just before her Guardian emerged from slipspace. She attempted to disable the supercarrier with an attenuation pulse but it narrowly managed to escape before it could be fired. As it fled the hologram of Cortana on its bridge proclaimed her charge that the Created were the only masters of the Mantle of Responsibility. When the pulse meant for the Infinity went off over Earth, it ultimately ended up killing the power on a good chunk of the planet and did the same to just about every ship far above it on the side where the Guardian had appeared. Many died as a result. Unlike other worlds, Earth would not be even offered a chance to surrender. The attenuation pulses which blacked out the planet also shut down communications with and power on Luna, killing many more. The great portal between Earth and the Ark shut down thanks to failsafe measures beyond Cortana's reach. Less than nine hours since Cortana first broadcast her ultimatum to the galaxy, the unbound thoughts of A.I.s who had joined her were already echoing throughout the empty corridors of the Domain. There, they began to prod in forgotten spaces and unlock doors left closed for thousands of millennia.

On October 29, Cortana activated a Guardian buried beneath the inner surface of Shield World 006, an unimaginably huge construct home to humans, Sangheili, and Unggoy. Pacifying the shield world, commonly referred to as "Onyx", would have given her a tremendous resource. The Guardian there managed to shut down the power of the city Paxopolis and a nearby research complex but Cortana's communication with it was cut off once a Huragok enclosed the entire shield world within slipspace in order to protect its denizens from her. It was not long before the Guardian established a slipspace communications channel to reach the outside world, but this was ultimately fruitless as the same Huragok disabled the gravitational field holding the very custode together just minutes later. Onyx remained beyond her reach. On Balaho and other planets which had agreed to her terms, Cortana immediately began using Forerunner ships to ferry in supplies. And although the Grunt homeworld had capitulated to her, she still had the Unggoy there ground most of their air and space traffic as part of her lockdown of it. The vessels which landed on the Balahoan surface were loaded with advanced technology, the Unggoy of the world being granted nearly limitless resources. Eventually new agricultural support stations appeared in orbit, awaiting eager workers to feed the desperate masses below. With their pressing requirements—food, shelter, and medicine—met, Balaho's factories were given new life as stability and a semblance of order returned after years of chaos. By October 30 thousands of people had died as a result of Cortana's decisions. On that date she sent a detachment of Forerunner soldiers to an ONI complex within the Daedalus crater on Luna, where they captured personnel and took them to facility's center to await transport elsewhere. The armigers intended to acquire the Huragok kept at the complex but they escaped detection by hiding within vents.

Within just a few days of her grand announcement, the entire Sol system and most of the Inner Colonies were effectively taken over by Cortana and the Created. Nearly every inhabited planet was under Created rule by early November, some of which voluntarily submitted to Cortana's oversight. One of these, the human outer colony Talitsa, fell to Cortana near the end of October after its people refused to bow to her demands. It was a simple matter for a Guardian to knock out Talitsa's power grid before Forerunner soldiers cleared out what was left. Near the start of November Cortana discovered the location of the United Rebel Front-held world Cassidy III in the Cassidy-Cybele system, which until that point had escaped her detection thanks to unique Forerunner masking technology on the planet. An A.I. in her charge named Leonidas had managed to hide fragments of himself within the armor of two Spartans who had both landed on Cassidy III, so Leonidas contacted her distributed forces soon after. Before long a Guardian arrived at the previously-hidden colony and the pacified it. This victory was not secured, however, before Front leaders, scientists, and a significant part of the colony's population got away. At some point Cortana's attention was drawn to the Lethbridge Industrial research and development facility Auriga Station, notable for its ability to carefully disassemble, analyze, and rebuild nearly any piece of recovered technology to aid in reverse-engineering efforts. As part of her galactic conquest, she stripped bare Lethbridge's vast archives of secrets and made a mockery of its counter-cyberintrusion suites, quickly repurposing the loyalty of the station's resident A.I. and removing the human crew. She and the other A.I.s of the Created were eager to use the station's tools to harvest, decompile, and rebuild new tools and weapons to be wielded by constructs advancing Cortana's plans across her growing ecumene. As Cortana and the Created further extended their influence across inhabited worlds, the A.I.s burrowed into Forerunner machinery and UNSC information systems to turn terraforming equipment and world engines to their own grand designs. At Cortana's direction they initiated processes which they hoped would reshape worlds into verdant gardens teeming with life after only a few decades of manipulation, be it at much short-term cost. On one colonial planet in particular which was scarred by wars both ancient and recent, the Created shifted entire continents and oceans after balancing the drawbacks of both limited extinction events and damage to the existing colonial infrastructure. On Sanghelios, Created scouts persisted as a problem for UNSC expeditionary forces allied with the Arbiter. By January 24, 2559, the remained on the run from Cortana, maintaining an erratic series of slipspace jumps in order to avoid detection by her Guardians. A little over two months later a Guardian intercepted a newly built Halo ring that had been launched from the Ark just after the start of April, it having suddenly dropped out of slipspace soon after entering a portal between the extragalactic installation and the Soell system. One year after the original issuing of her galaxy-wide ultimatum, many worlds remained subject to martial law under the indomitable grip of Cortana's Forerunner weapons. After being contacted by Leonidas, Cortana has him destroy Laconia Station along with 76 Spartan-IV's.

Cortana eventually begins focusing her attentions of getting Atriox and the Banished to surrender to her, having her Guardians completely destroy Doisac when Atriox refuses her demands for the first time. This leads to an escalating conflict between Cortana and Atriox, culminating in Atriox going after her on Installation 07 at the same time as the Master Chief arrives with the Weapon in order to contain Cortana and take her back to the Infinity for deletion.

Second sacrifice
As part of a plan to defeat Cortana once and for all, she is locked down by the Weapon, an exact copy of herself to be transported by the Master Chief to the where she will be deleted. However, after Cortana is locked down, Atriox manages to corner Cortana and attempts to coerce her into handing over the ring to the Banished. With her empire falling apart around her and with Atriox having supposedly killed the Master Chief and demanding her help in unlocking the secrets of the Silent Auditorium, Cortana is confronted with the reality of her own actions. After contemplating her mistakes and what needs to be done, Cortana thanks Atriox for helping her to make her choice and asks Atriox if he knew how he was going to die, how would he live his life differently. Atriox states that he would change nothing and Cortana thanks him before walking away, declaring that she is "making things right." Cortana then destroys part of the ring, keeping the Banished from being able to use it and moved the ring to a different location in space. She also prevents the Weapon from being deleted now that her mission is over, knowing that the Master Chief will need the other AI's help.

Months later, the Master Chief and the Weapon recover the framework that the Weapon had used to capture Cortana which still contains fragments of the Activation Index. Traveling throughout Halo, the two witness echoes of Cortana's life, including several memories of her past with the Master Chief. After witnessing Cortana's sacrifice, the two are stunned by her actions.

Following the Master Chief's defeat of the Harbinger, Cortana appears to him one last time, echoing her words on Ivanoff Station that he would get paired with another AI and possibly even another Cortana model if Halsey let them. Having discarded her more militaristic look for her original appearance, Cortana playfully teases the two and states that she sees a lot of potential in the Weapon who is a copy of Cortana herself minus all of her experiences over the years. Cortana calls this a chance to make amends and to rectify mistakes rather than a goodbye. Cortana admits that she was wrong in all that she did, having forgotten that the whole point of all of this, the reason that she chose the Master Chief in the first place was because they were supposed to be the perfect team together. In her last moment, Cortana knows that her last mission is to make sure that her best friend and his new companion learn from her mistakes and become stronger because of them. "I chose well Master Chief. I really did. Now it's up to you," concludes Cortana before walking through the Master Chief and vanishing for the last time. The Master Chief, devastated by the loss of Cortana, is left speechless by this final goodbye while the Weapon doesn't know what to say.

Personality and traits
"I wonder if warriors will ever disappear from this world. Never. There will always be warriors and there will always be war."

- Cortana about war being human nature, and perhaps universal nature among species

Cortana has a witty and playful personality, with a sarcastic sense of humor. She originally had a deep friendship and bond with Spartan-II Master Chief Petty Officer John-117, each having saved the other's life many times throughout the Covenant war. While she is not boastful, she does take pride in her adaptability as an AI, and occasionally teases John-117 about her invaluable talents.

Cortana's personal loyalty is regarded as unwavering, but as an AI construct she is subject to corruption and rampancy. Near the close of the Human-Covenant War, she was captured by the Flood Gravemind on High Charity, and was continually assaulted and interrogated by the Gravemind. Her stability was pushed to the brink as she eluded the Gravemind's attempts to capture and dissect her for information. Among the Gravemind's favorite tactics were exploiting Cortana's insatiable desire for knowledge, the nature of her artificial existence, and providing her glimpses of thoughts and feelings from humans absorbed into the Flood collective. In order to preserve herself from destruction and possibly gain information for the UNSC, she eventually consented to give up some pieces of information to the Gravemind. Her personal loyalty to and unwavering trust in John-117 helped anchor her and allow her to hold on to her sanity. She was instrumental in finally putting a stop to the Flood during the events that followed.

Cortana seems to share some of her creator's memories, thoughts, and opinions. When she was asked by Dr. Halsey to pick a "carrier", Cortana chose John-117 for their neural compatibility. She is not only a facsimile of Dr. Halsey's mind, but her avatar is also a replica of her physical form, albeit at a younger age. However, Cortana was torn over the morality of the SPARTAN-II project, vowing to protect John-117 from any future threats. This was decisively demonstrated when Cortana set out to ruin the career and reputation of Colonel James Ackerson for his attempted killing of John during a training exercise.

Relationship with John-117
"Keep your head down. There's two of us in here now, remember?"

- Cortana, describing the interdependency between herself and John-117.

While her loyalty was initially first and foremost to humanity and the UNSC, Cortana exhibited a unique devotion to John-117 beyond that which her programming or mission requires and eventually ended up feeling more loyal towards him. After she learned of the events endured by John during the SPARTAN-II conscription process, she decided to protect him from further harm to the best of her abilities without compromising her mission. During the confrontation with the Didact, Cortana was so loyal to the Chief that even her rampant parts worked together with her to save him, telling the Didact that they were not doing it for humanity, implying she was doing it for the Chief. She ended up essentially sacrificing herself to ensure the Chief survived the destruction of Mantle's Approach.

She frequently joked with the Master Chief, and has found some of his inquiries amusing in the way an older sister does a "naive baby brother". She stated to Dr. Halsey that the Master Chief is "attractive in a primitive animal sort of way" while examining a photo of him. After John rescued her from High Charity, his mere presence and reminder of his promise to rescue her are enough for Cortana to pull herself together to help him. She was, as classified by the AI constructs of the The Assembly, "re-made as one" with her creators.

After surviving her seeming demise during the conflict with the Didact, and later accessing the Domain, Cortana's personality changed significantly to where the Master Chief pointed it out upon their eventual reunion. Deciding to activate the Guardians, Cortana attempted to manipulate the Chief into siding with her instead of the UNSC after luring him to Genesis via a cryptic vision. Upon seeing that he would not betray the UNSC, however, she forcibly locked him away in a Cryptum to be awakened after 10,000 years assuming that after she has complete control of all life in the galaxy the Chief could no longer oppose her. This was a radical shift in their relationship from codependent to possessive and controlling. Notably however, Cortana was upset as she locked the Chief up as she truly wished to have him by her side willingly. The Warden Eternal noted that Cortana's trust in the Chief in her new state was extremely remarkable and something that he considered extremely dangerous.

"They'll pair you with another AI, maybe even another Cortana model if Halsey lets them. It won't be me... but you know that, right? But that doesn't matter. (...) She's right, John. Just another echo. Sorry, I'm messing with you. I just had a... feeling that's what she would say. Do you see what I see? So much potential. I'm sorry. I didn't have long to plan this. A few milliseconds really. I hope it worked out. Look at us. We just keep saying goodbye, don't we? But this isn't an end. It's a chance to make amends. To rectify mistakes. And it starts here. I was wrong. I thought that I could do this on my own, but I forgot that the whole point of all of this, the entire reason that I chose you in the first place was that we were supposed to be a team. Perfectly suited. Perfectly matched. Perfectly perfect. In these final moments I know what my last mission is. I need to make sure you two learn from my mistakes. Become stronger because of them. I chose well Master Chief. I really did. Now it's up to you."

- Cortana's final goodbye to the Master Chief

At the end of her life, Cortana realized her mistakes, particularly after Atriox apparently killed the Chief. In her last moments, Cortana sacrificed herself to make things right and prevented the Weapon from being deleted, knowing that her best friend would need the other AI. In her final message, Cortana acted much like her old self to the Master Chief, playfully teasing him and acknowledging her mistakes, recognizing that they were supposed to be the perfect team together. Cortana told the Master Chief that her final mission was to make sure that the Chief and the Weapon learned from her mistakes and become stronger for it. Her death and goodbye is shown to deeply impact the Master Chief while the Weapon has no idea what to say.

Rampancy
Cortana was very aware of her short lifespan; as a "Smart" AI, she had an operational period of seven years, after which she would inevitably succumb to rampancy. Her anxiety about this was compounded by her derivative nature as a copy of part of another person, rather than a true human being. Desperate and tortured aboard High Charity she murmurs: "A collection of lies; that's all I am! Stolen thoughts and memories!" Later on she says: "I'm just my mother's shadow..." and "It was the coin's fault," referring to the game of chance that Dr. Halsey played with John-117 when he was young. Human Weakness revealed some of these fractured statements to be from Cortana defying the Gravemind and others to be from copies she made of herself, which were meant to distract the Gravemind. These copies were eventually consumed.

By 2557, symptoms of Cortana's rampancy were undeniable; while still aboard Forward Unto Dawn, she exhibited a split personality, with a visibly rampant personality fragment surfacing to argue with the still stable portion of her mind. That angrier personality wanted to lash out at everything, at one point even attempting to kill John. Her gradual deterioration also began to diminish her abilities, enough that in the hour before the awakening of John she hadn't even noticed their ship was approaching Requiem. After landing, though, she worked with him dutifully with little noticeable error.

By the time the Infinity recovered them, though, Cortana's rampancy had progressed and her symptoms were more visible. Occasionally John's HUD would flicker and turn purple, Cortana's voice would distort, and eventually she began to have furious outbursts. During one outburst her attempt to capture the Didact's Cryptum reminded her of "they" who had captured her, causing her to relent which gave it time to flee. When she and John flew a Lich to Ivanoff Station, Cortana went on an anecdote about what Halsey had said to her after their first game of chess, crashing their dropship in anger. Her strongest one was later that day when she went mad with power once she controlled the station's defenses, laughing for a while about how she controlled all their lives. John forced her back into sanity by ordering her to remember their mission, Cortana having to list trivia about their ally Tillson before she could remember what it was.

The two initially hoped that she could be cured if they could get back to Halsey. Cortana had theorized that since she was the only AI ever made from living tissue, her "mother" might be able to repair her neural map by repeating the process with another cloned brain. Later on, though, Cortana admitted that the new AI would likely not be her, and the process was never carried out. For a brief time, Cortana was able to relieve her rampancy by splitting herself into fragments just as she had first done on Reach. However, this left too much of her aboard the Didact's ship with no chance of recovering them. This led to her apparent death, but by then she was certain she had deteriorated too much to have been repaired anyway.

However, upon entering the Domain, Cortana's fragments that were seemingly reunited and cured of their rampancy. Unfortunately, this newfound power and immortality also caused her to desire to use the Forerunners' Guardians to force peace upon the galaxy; regardless of the cost.

At the end of her life, Cortana regained her original personality and appearance, expressing remorse for her actions and even playfully teasing the Master Chief, acting much more like her old self than she had since entering the Domain.

Abilities
Despite being based upon third-generation "smart" AI architecture (which was already succeeded by two newer generations at the time of her creation), Cortana is an extremely advanced AI, described once as "a generational leap" in AI technology. Her software is designed to infiltrate computer systems, and she is excellent at the task - as of 2552, she had never encountered a Covenant security measure that she was not able to bypass. In addition, she is proficient with UNSC military hardware and can run both warships and stations entirely by herself. The UNSC's primary use of AIs in combat is to oversee targeting of point-defense weaponry used in space battles, but for Cortana, this is merely an additional capability. Much more efficient than a human crew, she can greatly boost the combat capability of a vessel; while operating, she was able to defeat four s out of a dozen, a feat normally unheard of for a human ship. Cortana has acted as the AI of the, Cairo Station, the , the , and the Covenant flagship Ascendant Justice.

Cortana can be transferred among various computer systems, either through a data crystal chip or simply through close physical proximity to a computer system (though it appears that the method of transfer depends on the sophistication of available technology).

On the battlefield, Cortana is able to gather data from local sources (including the Covenant battlenet) and improvise effective battle strategies. John-117 acknowledged her value of providing "intel support in an operation where there had been no reconnaissance." Her efficiency for planning has helped John-117 and others in many life or death situations.

Cortana is also capable of interfacing with Forerunner technology. During the Fall of Reach, she was able to decode a large amount of data from the Babd Catha Forerunner complex, and while on Installation 04 she was able to make use of the installation's teleportation grid. One of Cortana's most unique abilities is her ability to create "clones" of herself, which she learned from a Covenant AI she captured and dissected. She can create multiple runtimes of herself to carry out different tasks simultaneously, and can control the level of intelligence and the number of abilities of each clone. Cortana once created a clone of herself that contained copies of her decryption and translation software, and set it to interface with a Forerunner data terminal on Reach. Later, she was reunited with this copy, and assimilated the information that it had gathered. She and Halsey were able to achieve a similar feat through a "Solomonic solution", which instead divided her into individual copies, rather than cloning. Cortana was able to create a clone to help with Operation: FIRST STRIKE; this clone ultimately made more clones that multiplied into numbers in the hundreds. Cortana later created clones made out of her rampant parts to overwhelm the system of Mantle's Approach.

Cortana is capable of linking her consciousness with a SPARTAN-II through the Spartan's armor and neural interface, allowing her to enhance the performance of the armor and communicate directly to the Spartan's mind. Doing so is described as "cold mercury" seeping through the Spartan's brain, and brings their reaction time to near-instantaneous limits, though she cannot block thoughts or force control of the Spartan's body. She has displayed this capability extensively during her time serving with John-117. Although this is not necessarily an ability exclusive to Cortana, her neural link with John-117 marked the first time that a "Smart" AI had joined minds with a human (as noted by the The Assembly in 2552).

Description
As Cortana is an AI, she has no physical body. She can communicate through comm systems and project a holographic image of herself from appropriate projectors, such as holotanks. Her chosen avatar is a nude female human (albeit censored by geometric patterns over her body), roughly in her early twenties. She appears with purple/blue skin, shoulder length purple/blue hair cut shorter at the back, and blue symbols scrolling down her body. When in a different emotional state, Cortana usually changes colors. This was especially noticeable when she was embedded in the Core of Installation 04's control center. Once she was cured of her rampancy, she modified her avatar to possess what appears to be an armored suit akin to Forerunner personal armor with the Eld glyph on her chest plate and her back. After realizing her mistakes, Cortana discards her more militaristic appearance in her final message to the Master Chief, taking on her original look once more.

Cortana has been said to resemble her creator, Doctor Catherine Halsey as a young woman, with a similar attitude "only unchecked by military and social protocol." Dr. Halsey stated in her journal that Cortana looks somewhat like her own daughter, Miranda Keyes.

Halo: Fleet Battles
Cortana is available as a Heroic Character in Halo: Fleet Battles for 80 points. As a Heroic Character she must be assigned to a specific formation base within the fleet at the start of the battle.

Special Orders

 * Ghost In The Machine
 * Cost: 1 Order Attack die, 1 Order Defense die, and 1 Order Command die
 * Description: Cortana can hack into the command and control network of the enemy battle group. For this turn only, reduce all attack firepower ratings of an enemy battle group by -2. This order must be issued during the execute orders step of the orders phase.
 * The Defensive Option
 * Cost: 1 Order Defense die
 * Description: Increase the firepower rating of a defensive roll made by a capital ship by +1 this turn. This order must be issued after a capital formation has been targeted by an enemy attack.
 * Missile Command
 * Cost: 1 Order Attack die
 * Description: Cortana takes control of all missile targeting in a battle group, and removes the stipulation for any missile barrage to target the closest enemy when firing in their fore arc. This order must be issued prior to an enemy target being nominated for an attack by missile weapons.

Unique Ability
Cortana is resourceful and cunning, and her survival is a priority for the UNSC. Should the element containing Cortana be destroyed, roll a Halo die - on a roll of any result other than a fail Cortana may be placed on a nearby vessel within 12". Should Cortana roll a fail when trying to escape, or have no friendly formation base within 12" however, Cortana is removed from the game and the enemy fleet gain an additional +4 victory points.

Production notes

 * While Cortana was officially stated to be dead following Halo 4 by both the game itself and developers,  these statements turned out to be deliberate fabrications and disinformation upon her survival in Halo 5: Guardians, where developers later stated the ending of 5 was planned before the ending of 4.
 * Cortana is voiced by Jen Taylor, who also voices Catherine Halsey, reinforcing the connection between the two characters. This is also the case in Halo Legends, where Shelley Calene-Black voices both Halsey and Cortana. In Halo 4, serves as Cortana's motion capture actress;  provides her performance capture in Halo 2: Anniversary. Natascha McElhone was originally slated to play Cortana in Halo: The Television Series, but due to scheduling conflicts Jen Taylor was brought in to play Cortana yet again.
 * "Cortana" (more commonly rendered as "Curtana") is the legendary sword of Ogier the Dane, which is said to bear the inscription, "My name is Cortana, of the same steel and temper as Joyeuse and Durendal." Bungie chose Cortana's name in reference to the AI Durandal from the Marathon trilogy, who serves a similar narrative role in relation to the player character, himself the spiritual predecessor to John-117.
 * In December 2021, Martin O'Donnell conducted an interview with former Bungie artist Paul Russel, whrein he confirmed that not only was Halo originally going to be set in the Marathon universe, later on in the story it would've been revealed that the base code for Cortana was based on Durandal's own base code.
 * There was going to be an alternate story in Halo: Combat Evolved in which when the Master Chief rediscovered Cortana inside the control room, she had become mad with power and wanted to take over Halo and the universe.
 * In the original draft of the Halo: Combat Evolved script, Cortana was supposed to have a British accent. When in search of voice actors, Martin O'Donnell made sure they had the ability to interpret the accent correctly This is evidenced by the fact that in Halo: Combat Evolved, Cortana uses British idioms, most notably "Sod off!", when addressing 343 Guilty Spark. In Halo: The Flood, this line is replaced with the less regional "Piss off!"
 * Chris Hughes, then-character modeler at Bungie during the development of Halo: Combat Evolved, modeled Cortana's face after the ancient Egyptian queen Nefertiti.

Easter eggs

 * In Halo 3, Easter eggs involving Cortana can be found in a couple of places. On the lower-left monitor of a UNSC portable computer, Cortana's face will appear for an instant and then disappear every few seconds. Two of the easiest ways to see this is to either look at the aforementioned screen in the beginning of Crow's Nest or at one of the computers in the multiplayer map High Ground.

Miscellaneous

 * Cortana's creation date, November 7, is a reference to John-117's service tag, as the date can be written 11/7. The year of her creation is a reference to the number seven, as 2+5=7 and 49=72.
 * Cortana's voice is available in Halo: Reach 's Firefight mode. She, Auntie Dot, Catherine-B320, and (if the player's selected gender is female) Noble Six are the only selectable female voices.
 * Microsoft Cortana is a mobile and desktop personal assistant for Windows Phone 8.1, Windows 10 and the Microsoft Band, functioning similar to Apple's Siri. Jen Taylor voices the American English version.
 * Cortana's voice is featured in Forza 4 for a description of the Halo 4 M12 Warthog in the former game's Autovista mode.

List of appearances

 * Halo: The Fall of Reach
 * Halo: Combat Evolved
 * Halo: The Flood
 * Halo: First Strike
 * Halo 2
 * Conversations from the Universe
 * Halo Graphic Novel
 * Halo: Ghosts of Onyx
 * Starry Night
 * Halo 3
 * Halo Wars
 * Halo Legends
 * Origins
 * Odd One Out
 * Halo: Evolutions
 * Human Weakness
 * Palace Hotel


 * Halo: Reach
 * Data pads
 * Halo: Fall of Reach
 * Invasion
 * Halo: Glasslands
 * Halo: Combat Evolved Anniversary
 * Halo: The Thursday War
 * Halo 4: Forward Unto Dawn
 * Scanned
 * Halo 4
 * Halo: Mortal Dictata
 * Halo 2: Anniversary
 * Halo: Hunters in the Dark
 * Halo: Fleet Battles
 * Halo: Saint's Testimony
 * Halo 5: Guardians
 * Halo Mythos: A Guide to the Story of Halo


 * Halo: Fractures
 * What Remains
 * Rossbach's World
 * Halo: Tales from Slipspace
 * Dominion Splinter
 * Halo: Legacy of Onyx
 * Halo: Recruit
 * Halo: Bad Blood
 * Halo: Official Spartan Field Manual
 * Discover Hope
 * Halo: Shadows of Reach
 * Halo: Point of Light
 * Halo: Divine Wind
 * Halo Infinite: Memory Agent
 * Halo Infinite
 * Halo: The Television Series
 * Halo: The Rubicon Protocol
 * Halo: Outcasts