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Fireteam Osiris is a specialized combat and intelligence gathering team of Spartan-IV supersoldiers of the Spartan branch. The team carries out complicated and classified missions for the Unified Earth Government and its United Nations Space Command, typically dealing with issues that are deemed hard to control by the UNSC High Command. Fireteam Osiris' activities are classified at the highest levels of government.
Fireteam Osiris is composed of four of the most prominent and effective personnel from throughout the different branches of the United Nations Space Command. Osiris' members were selected based on their skills, with the desired outcome being that each team member's skills would mitigate another's deficiencies. The fireteam's balanced skillset and impressive success rates cause them to be commonly deployed in extremely sensitive and complex missions.
By October, 2558, Fireteam Osiris were assigned to UNSC Infinity.
Rescue and assassination
After several human colonies were unexpectedly attacked by mysterious Forerunner entities, Doctor Catherine Halsey contacted UNSC Infinity with information about the attacks. In response, Fireteam Osiris was tasked with rescuing Dr. Halsey from the custody of Covenant Supreme Commander Jul 'Mdama on the Forerunner world of Kamchatka.
After being briefed by Captain Thomas Lasky of UNSC Infinity, the Spartans were delivered to Kamchatka via D79-TC Pelican, where they discovered the Covenant engaging their former Promethean allies. Osiris exited the Pelican dropship to free fall past battling Covenant and Promethean ships, using their armor's thrusters to control their following landing. Osiris landed on a mountainside, where the Covenant and Prometheans were currently engaged in combat and the Spartans fought their way through the terrain. While Vale eliminated Covenant forces aboard a Mikpramu-pattern Phantom, Locke and the rest of Osiris battled numerous Covenant and Promethean forces on the ground. As one of the Covenant blockade runners crashed into a nearby mountain peak, Osiris regrouped and was forced to outrun the ensuring avalanche. The Spartans leaped off a cliff and slide down the mountainside, where they promptly engaged nearby Covenant forces in a valley. With 'Mdama and Halsey located in a nearby Forerunner structure, Fireteam Osiris fought their way past Covenant and Promethean forces to reach their targets' location. Although 'Mdama had a Shuul'se-pattern Kraken deployed to fend off the hostile forces assaulting his location, the Prometheans managed to force the walker to retreat.  As Osiris battled their way to the structure, they arrived at 'Mdama's and Halsey's position on a balcony. While Tanaka, Vale, and Buck engaged 'Mdama's Zealot guards, Locke charged towards 'Mdama and engaged the Supreme Commander in close-quarters combat. After a brief fight, Locke managed to assassinate 'Mdama, and Tanaka, Vale, and Buck quickly eliminated the remaining Sangheili soldiers on the platform. With Halsey recovered, Osiris and the doctor returned to UNSC Infinity.
Hunting the legend
Upon their return to Infinity, Fireteam Osiris learned that Spartan-IIs John-117, Kelly-087, Linda-058, and Frederic-104 of Blue Team traveled to the Outer Colony of Meridian—absent without leave—to investigate smart AI Cortana's location. Osiris was tasked with embarking to the recently resettled and now independent Meridian to track down Blue Team and take them back to the UNSC. To prepare for the mission, Locke tasked an AI to identify weaknesses in Spartan-IIs and the MJOLNIR Mark VI and acquired an armor-locking device that could short circuit their armor's systems. Fireteam Osiris arrived at Pinnacle Station—the Meridian space elevator—via Pelican, where they descended to the moon's surface. Osiris quickly discovered that the world was also under attack from Promethean constructs, with Meridian's settlers desperately attempting to fend off the invaders. In exchange for colony leader Governor Sloan's cooperation, Osiris agreed to help defend Meridian from the Prometheans. After destroying the Prometheans that were assaulting the outpost at the space elevator's base, the Spartans began travelling to Meridian Station, the lead settlement on the moon which the Prometheans were attempting to invade. However, Sloan was reluctant to give Osiris access to the road leading to the settlement, fearing that the Prometheans would follow. However, after Osiris eliminated the surrounding Prometheans, Sloan allowed the Spartans access to the road. At Sloan's request, Osiris aided several settlers under attack at a nearby service garage and Sloan ordered his men to give the Spartans one of the Liang Dortmund Corporation's M820 Scorpions to use against the Prometheans in exchange for their help.
After using the Scorpion to clear out the rest of the nearby Prometheans, Vale and the rest of Osiris proceeded to Meridian Station, where they discovered that Sloan was a smart AI suffering from onset rampancy. Sloan allowed the Spartans to search the settlement for clues about Blue Team's location, leading to Locke having Osiris split up to see if they could learn anything from the local settlers. After learning that Blue Team's prowler was seen traveling to the nearby Apogee Station, Fireteam Osiris boarded a transport Pelican and departed for the settlement. Upon arriving at Apogee, they found the settlement under attack. Working with Meridian soldiers, Locke and the rest of the Spartans cleared out the Prometheans and discovered Blue Team's abandoned prowler. Accessing the prowler's data banks, Osiris discovered that Blue Team had traveled to the mines under the settlement. After eliminating more Prometheans throughout the mines, Osiris discovered that the Meridian settlers had found a massive Forerunner structure underneath the moon's surface. As they investigated the structure, they eventually came across the Warden Eternal, an ancilla that served as the protector of the Domain and claimed his loyalty belonged to Cortana. After refusing to allow the Spartans to pursue Blue Team, Fireteam Osiris engaged the Warden and eventually destroyed him. Advancing forward, Osiris came across a Guardian in a large, cavernous chamber. While exploring the area, Vale discovered a local slipspace translocation network throughout the cavern and noticed Blue Team using it on above platforms. Following Vale, Locke and the rest of Osiris pursued Blue Team as they advanced from platform to platform utilizing the translocation network. During the chase, Locke got ahead of the rest of his fireteam and was teleported to the same platform that Blue Team was on.
As the Spartan-IIs prepared to travel through one last teleporter to reach the Guardian, Locke ordered them to stand down. The rest of Osiris was teleported to a higher platform, as John-117 ordered his own teammates to go through the portal ahead of him. When John refused to stand down and moved to follow his team, Locke attempted to detain him, leading to a hand-to-hand combat fight between the two while the rest of Osiris watched nearby. John eventually gained the upper hand and used Locke's own armor restraint against him. As John escaped through the portal and entered the Guardian, the massive construct began to activate and the floating platforms around Osiris began falling out of place and into the lava below. Freeing Locke, the four Spartans rushed back the way Blue Team came by traveling back to each platform via teleporter. Fireteam Osiris was just able to enter the last teleporter as its platform plummeted towards the lava, with Locke assisting Buck through the portal when the latter had nearly fallen off the platform. The Spartans were teleported back to Meridian's surface, where they discovered that the colony was already well into its evacuation as settlers rushed into the moon's space elevator. As Osiris raced to the elevator to escape the ensuring damage that would be caused by the Guardian's activation, the construct rose out of the surface nearby. After fighting their way to the base of the elevator, Sloan allowed Osiris to enter. Sloan then revealed that he was warned of the Guardian's activation by Cortana, and that he intended to be a part of the future Cortana was creating for humans and AIs. Osiris destroyed the remaining Prometheans nearby and, after activating it, the fireteam began ascending the space tether. On the way to the elevator terminal, while engaging more Prometheans, the Guardian sent out a shockwave that heavy damaged the elevator. The Spartans were forced to climb the elevator as the structure slowly began to fall. Just as Osiris arrived at the platform their Pelican was on, the Guardian continued to send out more frequent shockwaves that slowed the Spartans down and caused the platform they were on to collapse. Osiris barely managed to enter the dropship, with Locke having to use his armor thrusters to board the Pelican as it fell from the platform. Once Locke was aboard, the Spartans left Meridian's atmosphere as the Guardian entered slipspace and left the moon.
Alliance on Sanghelios
After Fireteam Osiris returned to Infinity, Lasky ordered for the fireteam—along with Commander Sarah Palmer and Halsey—to travel to the Sangheili homeworld of Sanghelios to utilize the planet's inactive Guardian to pursue Blue Team. Vale and the rest of Osiris were taken to the planet's surface via Kmiro'sish-pattern Lich, where they were tasked with finding Arbiter Thel 'Vadam while he was leading the Swords of Sanghelios against the invading remnants of 'Mdama's Covenant at the Elder Council Chamber in Nuusra. As Osiris engaged Covenant forces, Palmer had Mahkee 'Chava deploy two Mantises for the Spartans to utilize in clearing out the Covenant warriors at the Elder Council Chamber. Upon arriving at the chamber, Osiris battled the Covenant forces assaulting the building and fought their way inside, where they meet up with 'Vadam. With Osiris' help, the Swords of Sanghelios eliminated the Covenant forces at the Council Chamber. Moments later, 'Chava arrived in a Phantom and extracted the Arbiter and Spartans. Traveling to the Swords of Sanghelios' encampment at Nuusra, Osiris worked together with Halsey, Palmer, and 'Vadam to formulate a plan to utilize the Guardian. 'Vadam, recognizing Locke as the ONI agent that had planned to assassinate him in the Covenant War, was initially reluctant to move his forces to assist the Spartans. However, when Cortana briefly shorted out all electronics at the encampment, 'Vadam agreed to work with Locke. Osiris was tasked with locating a Forerunner Constructor at one of the Guardian's nearby support stations to allow Halsey to input commands into the construct to force it to activate the Guardian. Palmer flew Fireteam Osiris to the support station, but was forced to drop them off just south of the facility due to the presence of a Covenant Kraken nearby. While the Spartans provided ground support, the Swords of Sanghelios assaulted the walker from the air. While the Arbiter's flight squadrons forced the Kraken into retreat, Osiris fought their way to an ancient Sangheili temple located near the support facility. After defeating several Prometheans at the temple, the Kraken arrived at the temple, threatening Osiris' mission. As the walker neared, the Spartans boarded the Kraken and destroyed its power generator from within. Osiris fled that doomed walker and traveled into the support facility via stolen D'nomlhe-pattern Banshees. Once inside the station, Osiris retrieved a Constructor, but were attacked by Promethean forces. After battling their way back to an extraction point with the Constructor, a Phantom transported them back to the Swords of Sanghelios' encampment.
With the Constructor, Halsey inputted her commands into it and the construct took off to activate the Guardian. However, the Guardian was located in the Csurdon Sea next to the city of Sunaion, the last bastion of the Covenant on Sanghelios. The Arbiter's forces regrouped and prepared to attack the city to eliminate the Covenant and give Osiris a chance to board the Guardian. Traveling to Sunaion with a squadron of air vehicles, Fireteam Osiris, Arbiter 'Vadam, and several Swords of Sanghelios forces were deployed to Sunaion and engaged Covenant soldiers throughout. While the Arbiter led his forces against the Covenant directly, Osiris destroyed Covenant anti-air emplacements to allow the Arbiter's fleet to move in closer to the city. Unable to receive air support for transport to the Guardian, Osiris accessed Sunaion's undercity to travel to its location. When the Constructor activated the Guardian, Promethean forces arrived at the battle and began engaging the Spartans, as well as the Arbiter and his forces. When the Guardian began rising out of the sea nearby and started to activate its shockwaves, the buildings around the Spartans were ripped apart. As the Spartans moved to safety, they regrouped with the Arbiter as he was engaging Promethean forces. The five battled the onslaught of Prometheans, which were soon led by the Warden Eternal, now utilizing another physical body. After a lengthy engagement, the five destroyed the Warden and the nearby Prometheans just as the Guardian prepared to leave. While the Arbiter remained behind to finish eliminating Covenant and Promethean forces, Palmer and Halsey picked up Osiris in the Pelican and flew towards the Guardian. A pursuing Banshee damaged the dropship, though Palmer was still able to drop Osiris off on the Guardian's surface just as it entered slipspace and left Sanghelios.
Upon exiting slipspace, the Guardian arrived at the Forerunner world of Genesis, where all the other Guardians had traveled to at Cortana's demand. While the Spartans observed their new surroundings, the Guardian attempted to shake the Spartans off, forcing Locke and his team to quickly descend down the construct until they were close enough to the ground to safely jump off the Guardian. Osiris soon encountered monitor 031 Exuberant Witness, who served as caretaker of the installation. The ancilla revealed that Cortana was intending to use the Guardians to force peace across the galaxy and to wipe out those that refused, apparently trying to instill the Forerunners' Mantle of Responsibility on all species with herself and other AI—now calling themselves the Created—at its beholders. Osiris resolved to aiding Blue Team and stopping Cortana, with the help of Exuberant Witness. As the Spartans traveled to Blue Team's position at Genesis' Gateway, they encountered Covenant forces that were inadvertently brought to Genesis with the Guardians, as well as hostile Promethean forces. Osiris fought their way towards the Gateway, were they were attacked by the Warden Eternal once again but managed to defeat him. Upon arriving just outside the Gateway, Osiris came under attack from more Prometheans and several of the Warden's bodies, as they attempted to buy time for Cortana to finish her plans. After a long engagement, Osiris proved victorious and Exuberant Witness gave the Spartans access to the inside of the Gateway.
Upon entering, Osiris and Exuberant Witness encountered Blue Team. While the eight Spartans agreed to work together to stop Cortana from enacting her plans, the AI suddenly teleported Blue Team to near her position. Cortana ultimately placed Blue Team in a Cryptum with the intention of keeping them in stasis for ten thousand years while she initiated her plans. Osiris moved to free Blue Team and, upon exiting the Gateway, witnessed the numerous Guardians entering slipspace to begin enforcing Cortana's new order. As the Spartans engaged Covenant and Promethean forces while on their way to the Cryptum, Cortana revealed that hundreds of AIs had pledged their allegiance to her, including Governor Sloan. Reaching the Cryptum, Osiris had to destroy several gravitational cores keeping the Cryptum in place. As Osiris came close to freeing Blue Team, Cortana began to taunt the Spartans in hopes of demoralizing them, but to no avail. After destroying the gravitational cores, Osiris activated a terminal to lower the Cryptum. However, Cortana suddenly pulled the Cryptum to the Guardian she was preparing to leave on. As Cortana prepared to leave, she began to disconnect herself from Genesis' systems, giving Exuberant Witness an opportunity to take full control of the planet once again. To aid the monitor, Osiris activated two nearby auxiliary power stations to help Exuberant Witness regain control of Genesis. The monitor provided the Spartans a route to Cortana's Guardian and the Cryptum, and Osiris rushed to stop their departure, pushing past the Guardian's powerful shockwaves. Locke managed to reach the relay terminal that gave Cortana control and destroyed it, reinstating full control of Genesis to Exuberant Witness. As Vale came to Locke's aid, Cortana prepared to leave the planet aboard her Guardian with the Cryptum, but Exuberant Witness sent a fleet of Constructors to take the Cryptum from the Guardian just as it entered slipspace, freeing Blue Team. Following Cortana’s departure, both Blue Team and Fireteam Osiris together fought their way through hordes of Promethean constructs to reach a D79-TC Pelican that had previously arrived on the planet via a Guardian’s Slipspace entry. Agreeing to return to Sanghelios, the joint team requested Witness’ help, who opened a Slipspace portal for them.
Upon their arrival at Sanghelios, now mostly in the dark due to the Guardian attack at Sunaion, they correctly deduced they’d find their allies near Nuusra. Sure enough, they located the Swords of Sanghelios’ makeshift campsite in the cliffs and touched down, reuniting with Halsey, the Arbiter, and Palmer.
Soon afterwards, Buck was recruited to lead a secret ONI mission to contact the United Rebel Front on Cassidy III in order to learn more about the Forerunner technology cloaking their colony. Buck reformed his old team of Alpha-Nine for the mission while the rest of Fireteam Osiris, even Locke, were kept out of the loop on Buck's secret mission. Having regained his confidence in his leadership skills after the mission, Buck chose to leave Osiris to lead Alpha-Nine again on a permanent basis. After Buck's return, he found Osiris in the Infinity's Full Moon bar and they joked around together about what had happened since Buck was gone. When Buck and Captain Veronica Dare were married by Roland shortly after their return from the mission, both Alpha-Nine and Fireteam Osiris were in attendance.
Osiris was an Egyptian god of the afterlife, the underworld, and the dead.
List of appearances