Battle of Requiem

In December 2552, the aft half of the UNSC Forward Unto Dawn, carrying Spartan John-117 and AI Cortana, was transported via slipspace on a course toward Requiem after the last battle of the Human-Covenant War. Cortana activated Forward Unto Dawn's emergency locator beacon while the Spartan entered cryostasis. Within the next year, Jul 'Mdama led his forces, a partly reunited offshoot of the Covenant, to Requiem in search of the shield world's "treasure": he sought the Didact, the leader of the Forerunner military, due to their mutual hatred of humanity. However, 'Mdama could not find a way into Requiem, so the fleet held station in orbit. On July 21, 2557, the Dawn entered the planet's gravity well; 'Mdama sent his forces to board the frigate to keep any humans away. As Covenant troops entered the ship, Requiem began to scan the Dawn for a "Reclaimer".

Cortana awoke John-117 from cryostasis and informed him of intruders on the Forward Unto Dawn. When the duo found out that there was a Covenant fleet around the Dawn and one of the CRS-class light cruisers was moving into a intercept course, Cortana instructed John to fire one of Dawn's missiles at the cruiser. The Spartan battled through the wreckage and managed to destroy the cruiser moments before it fired its energy projector. While on the outer hull, John was scanned by a beam coming from Requiem, which then opened its interior for the Reclaimer. Forward Unto Dawn, along with several Covenant cruisers, was pulled into Requiem's interior.

John-117 and Cortana survived the fall into Requiem's middle shell. They decided to hijack one of the surviving Covenant ships to return to Earth, where they would be able to find Dr. Catherine Halsey to assess Cortana's rampancy. However, UNSC Infinity had picked up Forward Unto Dawn's distress beacon and was approaching the shield world. While looking for a way to contact Infinity, John and Cortana accidentally triggered Requiem's guardians, the Prometheans. The Prometheans attacked the Spartan as well as the Covenant. In an effort the warn Infinity of Requiem's gravity well, the Spartan-AI team disabled two energy pylons inside Requiem's core in order to eliminate interference emitted from a satellite in the core. When John attempted to contact Infinity in the satellite, it was revealed that the Didact had purposely manipulated Infinity's signal to trick John-117 into releasing him from his Cryptum. The Didact openly declared war on humanity and reassumed control of his mechanized Prometheans; viewing the Didact as a living god, the Covenant forces followed suit. John escaped Requiem's core just as Infinity entered Requiem's outer shell.

Infinity crashed into a jungle inside Requiem and was immediately attacked by Promethean and Covenant forces. During the attack, Infinity's commanding officer, Captain Andrew Del Rio, deployed Marines and SPARTAN-IVs for reconnaissance, believing that the Infinity would be able to fend off her attackers while providing support for ground forces. The ill-prepared Marines were overwhelmed by the unfamiliar Prometheans and suffered heavy casualties. In addition to losing their landing zones, many Pelican dropships were shot down. Infinity was eventually boarded, allowing the Didact to extract information about the Composer, a Forerunner superweapon, from the ship's mainframe. John-117 joined Commanders Thomas Lasky and Sarah Palmer just as Captain Del Rio recalled ground forces to Infinity. John cleared a landing zone for the UNSC forces and headed to Infinity's crash site. There, John-117 and a squad of SPARTAN-IVs broke through to one of Infinity's hangar bays. John piloted a Mantis assault mech and fought through Infinity's maintenance corridors to some of the ship's weapon systems. After John destroyed the Covenant jammers near the control terminals, Infinity disabled two CRS-class light cruisers and damaged the Didact's Cryptum, prompting the Forerunner to retreat. The repaired Infinity left her crash site, but since she was still under the effect of the Requiem's gravity well, Captain Del Rio sent John-117 and Gypsy Company to take down the Forerunner anti-air particle cannons near the gravity well generator.

Gypsy Company, aboard a Mammoth and led by Commander Lasky, destroyed the first and second anti-air particle cannons. However, a Covenant Lich heavily damaged the Mammoth. Even though John-117 and a SPARTAN-IV team destroyed the Lich, the Mammoth was forced to stop when it approached a Forerunner structure.

John entered the structure and, with Cortana's help, disabled the last two anti-air cannons. Cortana suddenly disappeared, so John searched the structure for her. He was taken to an archived personality of the Librarian, the Didact's wife and the one who saved humanity from the Halos. The Librarian informed John of the Didact's plan to imprison humanity into digital form with the Composer. In doing so, she altered John-117's genes, an action that would prove itself useful when the Didact's forces raided Ivanoff Station. John-117 returned to a UNSC convoy led by Master Sergeant Marcus Stacker and eventually reached Requiem's gravity well generator. Using targeting data provided by the Spartan, Infinity destroyed the generator.

Despite their explanation, Del Rio was skeptical, dismissing it as hallucinations from John’s age and Cortana’s rampancy. When he ordered the Infinity to return to Earth, Cortana gave a short outburst of rage followed by John leaving to stop the Didact alone. Departing from the ship with a Pelican gunship given to them by Lasky, the Chief set out to stop the Forerunner, almost succeeding but failing; the Didact managed to regain his ship, the Mantle's Approach and promptly left for Ivanoff Station to retrieve the Composer. He was followed by a fleet of Covenant Liches and the Master Chief who stowed away aboard one.

The Didact would arrive at Ivanoff Station and despite the efforts of the UNSC, he obtained the Composer and digitized everyone on the station excluding John-117, who was saved thanks to genetic augmentations provided by the Librarian. The Didact then set course for Earth, with the intent of digitizing its population, but was ultimately stopped and defeated by John-117. With the Infinity and the Master Chief gone, Requiem remained under Promethean and Covenant control. The UNSCDF would not return to Requiem until six months later, when the Infinity was again deployed to the shield world to establish research bases and obtain useful Forerunner technologies.