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- Rion Forge and her crew aboard the Ace of Spades discover the wreckage of the UNSC Roman Blue on Eiro.
- The Ace of Spades and her crew investigate the wreckage of the Rampant Perdition and rescue Ram Chalva from Gek 'Lhar on Laconia.
- Rion's crew faces off with Gek 'Lhar and his Sangheili in the debris field of the Etran Harborage. Cade is killed and what remains of the shield world's ancilla is recovered by the Ace of Spades crew.
- An accident involving experimental weaponry aboard Argent Moon causes a gaseous toxic substance to spread throughout the ship via its air systems, killing nearly all aboard. Those closer to the accident are killed instantly and, depending on distance from the accident, the rest of the vessel's personnel begin suffering symptoms from the chemical minutes or hours later.
- All Argent Moon crew is killed by the biological agent, with only the shipboard AI, Rooker, still active. As a result, ONI loses contact with Argent Moon and Rooker sends a message that warns any ONI personnel who board the station of the situation, then puts the ship on a course that keeps it away from all inhabited systems and jettisons what remains he can into open space.
May 6 to June 1
- Shadow of Intent participates in naval exercises with the UNSC Navy in a Joint Occupation Zone. After this the ship is tasked with the protection of Sanghelios and Sangheili colonies.
- At 0221 hours, the UNSC Taurokado arrives in the Korinth Prior system near the accretion disk created by the remains of Shield World 0459.
- At 0550 hours, After Action Report Q74S295-LR concerning the mission to the Korinth Prior system is sent by the artificial intelligence, Turkish, to Hugo Barton.
- UNSC Rubicon, an ONI vessel sent to reconnoiter Installation 00 in late 2553, is officially declared lost.
- Having tried in vain for three years, Jul 'Mdama's Covenant faction finally gain access to the shield world Requiem; Shipmaster Jul 'Mdama orders his forces to board UNSC Forward Unto Dawn to keep the ship from landing on Requiem. Aboard the Forward Unto Dawn, Cortana wakes John-117 from his cryotube and warns him of the intruders. Once off the frigate, they are forcibly pulled into the artificial planet.
- John-117 and Cortana engage Covenant forces on Requiem while attempting to find a way back to UNSC-controlled space. Meanwhile, UNSC Infinity is approaching Requiem, oblivious of the gravity well which pulled the Dawn and many Covenant ships inside. While attempting to clear the communications interference preventing them from contacting Infinity, John-117 and Cortana are attacked by mechanical Promethean forces. Activating a supposed broadcast relay in Requiem's core, John-117 instead accidentally releases the Didact from imprisonment. Bent on preventing human ascendance, the Didact leaves immediately and joins forces with Jul's Covenant.
- UNSC Infinity crashes on Requiem. The Didact makes for the crash-site, followed by the Master Chief, who reunites with the UNSC for the first time in years and assists in the defense of the stranded Infinity, eventually driving back the Didact and the Covenant forces. Afterwards, Captain Andrew Del Rio has a difference of opinion with the Master Chief on whether or not they should leave Requiem, or stay and deal with the Didact.
July 22-July 23
- Del Rio orders the Chief to join Gypsy Company and destroy Requiem's gravity well generator so that the Infinity can depart.
- During the mission, John-117 encounters a vision of the Librarian, who warns him of the Didact's true plan: to use a machine known as the Composer to neutralize humanity once and for all. To fight the Didact, the Librarian provides the Spartan with additional genetic enhancements, including an immunity to the Composer. Once the gravity well is destroyed, Captain Del Rio orders the Infinity to leave Requiem immediately. Disobeying orders, the Master Chief stays behind in an effort to stop the Didact.
- After the Infinity returns to Earth, FLEETCOM immediately relieves Captain Del Rio of his duties as captain of the ship due to his actions on Requiem; Thomas Lasky is promoted in his place.
- Between July 22 and July 24, the Didact repeatedly contacts Catalog and requests authorization to take control of the ecumene; Catalog rejects his requests.
- Raid on Ivanoff Station: John-117 pursues the Didact as he travels to Ivanoff Station to acquire the Composer. The Didact is eventually successful and uses the Composer to digitize the occupants of Ivanoff, save for John-117 who is saved by the Librarian's enhancements.
- SPARTAN-II Black Team, assigned as security for a science camp on Installation 03, loses contact with Ivanoff Station immediately after the attack.
- New Phoenix Incident: As the Didact plans to attack Earth and wipe out humanity using the Composer, John-117 follows him in close pursuit. When the Didact reaches Earth, the Infinity and the UNSC Home Fleet engage his ship immediately. Armed with a nuclear device, John-117 makes his way into the ship and confronts the Didact, but not before the Forerunner uses the Composer to digitize the entire population of New Phoenix, URNA. With the aid of a rampant Cortana, the Master Chief is eventually able to defeat the Didact, who falls into a slipspace rupture generated underneath the Composer. The Master Chief then destroys the ship with the nuclear device. Cortana sacrifices herself to save John-117 from the explosion and the Spartan is recovered from the debris field soon after.
- Unknown to everyone, Cortana survives with the debris of the Mantle's Approach and is pulled through slipspace to the Forerunner installation designated Genesis. There, she gains access to the Domain which "cures" her rampancy. Cortana meets with the Warden Eternal and both begin to plan to take the Mantle of Responsibility for the AIs.
- After falling into the Composer's slipspace rupture, the Didact is thrown to the surface of Installation 03 along with other detritus from Mantle's Approach. On Gamma Halo, the Didact awakens and brutally kills Team Black and the nearby UNSC science team.
- A service portal opens between the Composer's Abyss on Installation 03 and the Composer's Forge. The latter's monitor, 859 Static Carillon, attempts to contact the Halo's caretaker, 049 Abject Testament, but receives no response. Static Carillon travels through the portal to Installation 03, where he meets the Didact who asks to be taken to the Composer's Forge.
- John-117 is debriefed by the UNSC Security Council about the Didact and the loss of Cortana. The Council decides to tell the public that the attack was carried out by the Covenant remnants and it was stopped by Master Chief.
- Fleet Admiral Terrence Hood informs John-117 about the loss of contact with the Installation 03 science team an hour earlier, and that the attackers may have been Prometheans. John reunites with Blue Team — Fred-104, Kelly-087 and Linda-058 — and deploys immediately to Gamma Halo.
- After arriving at Gamma Halo, Blue Team find the bodies of the deceased science team and Team Black. After a short firefight with Promethean Knights, Blue Team discovers the former location of the Composer. Upon entering the structure, the Spartans discover the Composer's Abyss, at the end of which they find the slipspace portal leading to the Composer's Forge.
- Blue Team enter the portal to the Composer's Forge where they discover six Composers. The Didact confronts the Spartans along with a newly-created army of Promethean constructs.
- During the ensuing battle, Static Carillon transports Installation 03 above the Composer's Forge as part of an agreement between himself and the Didact. The Didact takes one of the Composers and departs for Gamma Halo, planning to use the ring to wipe out humanity. However, the monitor feels betrayed by the Didact bringing his "abominations" to the Composer's Forge and helps Blue Team return to the portal in pursuit of the Didact, intending to close the portal behind them.
- As Blue Team returns to the surface of Gamma Halo, they are attacked and easily subdued by the Didact. Before the Forerunner can execute the Spartans, 859 Static Carillon appears, disables the Didact and teleports him into the Halo's control room. John-117 follows the Didact into the control center where he enables Static Carillon to eject the segment of the ring housing the control room. The detached section of the Halo plunges into the Composer's Forge; the Didact is consumed by the ensuing blast while the Master Chief is safely teleported aboard Blue Team's Longsword in orbit.
July 24-July 26
- Battle of Installation 03: Covenant devotees assault Installation 03 and take control of the site known as the Altar in order to use the Conduit to enable a massive Promethean invasion of the Halo through slipspace portals. However, a Spartan Headhunter assaults the Altar and obtains the Conduit.
- Battle of New Phoenix: Shortly after the composition of New Phoenix, the Covenant open portals in an ONI base in the city through a Forerunner artifact stored there with the intent of launching an all-out invasion of Earth. The UNSC is able to stop the attack using the Conduit.
- 859 Static Carillon takes Installation 03 to an unknown location for repairs.
- Blue Team returns to Earth; John-117 is debriefed by Admiral Hood, who congratulates him on defeating the Didact. However, the Master Chief believes the Didact is not dead, but merely "contained". While Hood suggests that he and Blue Team stand down and have some rest, the Master Chief instead leads the team on a new mission.
- Rooker begins to suffer from rampancy due to being six months past his "expiration date". Feeling he can no longer manage the Argent Moon, he initiates his own destruction.
- Two months after Cortana's supposed death, Beta-5 Division operative Sapphire Three interviews John-117 regarding his visions of and interactions with the AI in late 2552.
- Gek 'Lhar carries out a terrorist attack on Earth; he is detained and sent to Midnight Facility for interrogation.
- An ONI analyst sends a warning memorandum to UNSC High Command regarding growing threats of Kig-Yar pirates near key trade routes. The UNSC disregards ONI's repeated requests for official action against the Kig-Yar.
- The CYPHER-class Mjolnir enters mass production by Imbrium Machine Complex.
- A Composer is extracted from Installation 03 by the UNSC Infinity and taken to Ivanoff Station.
- The Ruptive skin for the M395B DMR, MA5D assault rifle, BR85N battle rifle, M20 SMG, and M6H2 magnum is approved by the UNSC Ordnance Commission.
- The M820 Scorpion main battle tank is introduced.
- The Eden Rising, transporting a UNSC research team, arrives at Installation 00 via the Forerunner portal in Voi. Soon after their arrival, the UNSC establishes a new outpost and activates dormant Retriever Sentinels in order to start repairing the damaged Ark.
- ^ Halo: Fractures, Into the Fire, page 289
- ^ a b Halo: Smoke and Shadow
- ^ a b c d Halo Waypoint: Catalog Interaction - Page 14
- ^ Halo 4, The Commissioning
- ^ Halo 4 Limited Edition — Infinity Briefing Packet
- ^ a b c Halo 5: Guardians, campaign level, Blue Team - Mission Intel
- ^ a b Halo Legendary Crate, Data Drop #4
- ^ Halo Waypoint: Halo 4 Terminal: Jul 'Mdama
- ^ Halo 4, campaign level Dawn
- ^ Halo 4, campaign level Forerunner
- ^ Halo 4, campaign level Infinity
- ^ Halo Legendary Crate - Data Drop #7
- ^ a b Halo 4, campaign level Reclaimer
- ^ a b Halo 4, campaign level Midnight
- ^ Halo 4, campaign level Composer
- ^ a b c d Halo: Escalation - Issue #8
- ^ a b c d e Halo: Escalation - Issue #9
- ^ Halo 5: Guardians, campaign level Reunion
- ^ a b Halo 4: The Essential Visual Guide
- ^ a b c Halo: Escalation - Issue #10
- ^ Halo: Spartan Strike, Operation C: Valiant Hammer
- ^ Halo: Spartan Strike, Operation D: Adamantine Shield
- ^ Halo Legendary Crate, Data Drop #9
- ^ Section 3 Old Chat Record Page 99
- ^ Halo Waypoint: I wish 343i would of done more with Gek Lhar
- ^ Halo: Escalation, Issue #12
- ^ Halo 5: Guardians, CYPHER-class Mjolnir (Body description)
- ^ The Sprint: Season 3 - Episode 5
- ^ Halo Waypoint: Scorpion
- ^ Halo Wars 2, Phoenix Logs, "Henry Lamb Research Post"