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• February 28: The Template:UNSCship is commissioned into service.

• July 21: Having tried in vain for three years, the Covenant remnant 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 Requiem.

• July 21-Unknown date: 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 the Covenant. The Chief and Cortana narrowly escape the destabilizing core, arriving to the surface of Requiem just as Infinity crash-lands nearby.

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. In the end, Del Rio orders the Chief to join Gypsy Company and destroy Requiem's gravity well so that the Infinity can depart.

During the mission, John-117 encounters a vision of the Librarian, who warns him of the Didact's 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.

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.

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.

Battle of Earth: 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 population of New Phoenix, URNA. With the aid of a rampant Cortana, the Master Chief is eventually able to defeat the Didact and destroy his ship. Cortana sacrifices herself to save John-117 from the explosion and the Spartan is recovered from the debris field soon after.