Shield World 000, also known as Requiem, was part of the array of shield worlds built by the Forerunners prior to the Forerunner-Flood War. Requiem functioned as the primary shield world and home the Ur-Didact, the supreme commander of the Forerunner Warrior-Servants prior to his exile.
After the firing of the Halo Array, Requiem functioned as the home of the Ur-Didact, who remained locked in stasis inside his cryptum until his accidental release by John-117 during the Battle of Requiem. Once out, the Ur-Didact left Requiem, leaving the shield world to the only remaining faction on Requiem, Jul 'Mdama's Covenant. 'Mdama and his loyalists remained on Requiem, looking for access to Forerunner technology until the return of the UNSC Infinity in 2558, starting the Requiem Campaign. During this time, the crew of the Infinity battled 'Mdama's forces on Requiem for control, resulting in Requiem being sent hurtling into the host star Epoloch.