The Battle of Requiem was a conflict between former Covenant forces under the command of Jul 'Mdama, UNSC forces from the UNSC Infinity and Forward unto Dawn, and Promethean forces under command of the Ur-Didact on Forerunner Shield World 0001, also known as Requiem, in July 2557. The conflict initiated when Jul 'Mdama's fleet, which had been in orbit around Requiem since its discovery in 2554, came across the back half of the Forward Unto Dawn, which had been set adrift since the events of the Battle of Installation 00 in 2552. The presence of a human, John-117, considered to the Forerunners as the Reclaimers of the Mantle of Responsibility, triggered the automated systems of the shield world to scan the Forward Unto Dawn and open the entrance to Requiem. Both Covenant forces and the Forward Unto Dawn were drawn into the shield world's interior, beginning the battle.
The battle began on Requiem's interior surface, with the Covenant forces trying to take control of the Promethean forces on the facility, and John-117 searching for a way out and contact with the UNSC. A signal from the UNSC Infinity appeared to derive from the core of the facility, and in the ensuing search for the source, John mistakenly released the Ur-Didact, a Forerunner Warrior-Servant from his cryptum. This action brought the facility's security forces to full alert under the command of the Ur-Didact, and also forced the UNSC Infinity, which had actually been in orbit around Requiem, to crash-land on the shield world's interior.
The ensuing battle pitted the UNSC and John-117 against the Didact's forces in an attempt to escape back to human-controlled space. While attempts were made to keep the Didact from escaping as well, the Ur-Didact also managed to get to his ship, Mantle's Approach, and leave Requiem. The Infinity did manage to jump back into slipspace for Earth, but the Didact headed for Installation 03, leading into the Raid on Ivanoff Station.
These events are depicted in the video game Halo 4.