Requiem

Were you looking for the Halo 4 achievement?

Requiem, unofficially named the "legendary planet", was a Micro Dyson Sphere‎ in which the Didact was imprisoned inside by the Librarian. The massive exterior shell of Requiem encases a planet - a variation on a Forerunner Shield World. The, containing John-117 and Cortana, is seen drifting towards in the Legendary epilogue of Halo 3, and later crashes into after the first level of Halo 4.

Background
Requiem was the Ur-Didact's base of operations during both the Human-Forerunner and the Forerunner-Flood wars. It was the first and greatest of his Shield Worlds. From here, he searched for a way to immunize himself against Flood infection. When his efforts failed, he decided to use the Composer to convert Humans into Prometheans. The Librarian opposed his plans and imprisoned her own husband within Requiem.

First Battle of Requiem
The Didact along with an army of Promethean Knights are sealed away inside of Requiem that were unwittingly released by the Spartan John-117 and the A.I. Cortana; whom thought that they were trying to prevent the from being pulled into Requiem's gravity well like they were earlier, they traveled to the interior of the planet where they met resistance by Prometheans and the Covenant Remnant forces as they tried to turn off two relays aimed at the planet's core that was distorting transmission signals from being broadcast off of the Shield World. After turning off the relays, John-117 approached the core where they would be able to communicate with the Infinity clearly, only to see the interior of the core open up as a tall, pale humanoid appeared from within and assembled body armor around himself before standing up to see Covenant Remnant Elites kneeling before him as he waved his hand; causing the Prometheans blue lights to turn to orange ones before manipulating John-117 to float towards him and declare "Time was your ally, human. But now it has abandoned you... the Forerunners have returned." The Didact then activated a Slipspace rupture which brought his Cryptum to the surface of Requiem, along with pulling the UNSC Infinity to the surface and deploying his forces to assault the human vessel while he used his Cryptum to scan Infinity for knowledge. After a short assault by the Infinity, The Didact prepares to leave Requiem, having acquired the knowledge of the location of a Forerunner weapon once belonging to him. Despite the efforts of the Master Chief and Cortana (mostly due to Cortana's growing rampancy), The Didact succeeds in leaving with a Covenant fleet to Installation 03 followed by the Master Chief in a Lich.

Second Battle of Requiem
Six months after the Master Chief defeated the Ur-Didact, the Covenant maintained a presence on Requiem. The UNSC dispatched Infinity to clear out the Covenant and Promethean drones in order to set up research bases. Early in the battle the UNSC would unearth a Forerunner artifact that would cause them a great deal of trouble over the course of the coming days, including the loss of power on Infinity and later its immobilization over Requiem. Requiem was eventually destroyed when Jul 'Mdama activated the Shield World's fail-safe, pulling the planet into the sun, causing a supernova with the intent of destroying Infinity along with it. Thanks to the efforts of Spartan fireteams, Infinity would escape almost unscathed, although Requiem would be destroyed along with its sun; Epoloch.

Trivia

 * It is noted that in one of the posters the hold of the Forward Unto Dawn, the entrance varies in one having doors in a circular pattern that lift up to open whereas in the Halo 4 Commercial it is seen that it is a complex iris type door.
 * Requiem was the final piece written by the famous composer Wolfgang Amadeus Mozart. Its title meant "Mass (or song) for the dead".
 * Many Huragok know that Requiem is home to the Didact, however they do not know its location as a security measure.
 * Requiem is the second shield world to be destroyed by a star. It is also the 4th Forerunner installation to be destroyed (in-game) overall.
 * It is unknown if Requiem's shell rotates in sync with the planet itself or if it is independent. In the final cutscene of Spartan Ops, The outer shell can be seen rotating incredibly fast, but on the interior the fixed "sunlets" move across the sky, creating artificial days. This means that that either the shell moves and the planet itself doesn't, or Requiem rotates freely with the 'sunlets' actually changing position in the ceiling.
 * When Requiem collides against the star in its planetary system, it explodes creating a supernova. This is because Requiem being a Dyson Sphere (or in this particular case, a Dyson Shell) encompasses a star within itself and the collision between stars or objects of this mass and magnitude can lead to a core rupture of one or both stars causing a Supernova.
 * There is a possibility Jul 'Mdama didn't trigger the collision of Requiem but he triggered its release from an artificially held orbit around the system's star, and the gravitational pull between both "stars" or high mass celestial bodies would be too great to be overcome without external forces, thus their attraction lead to a collision and eventual destruction.

List of Appearances

 * Halo 3
 * Halo 4
 * Spartan Ops
 * Halo: The Thursday War
 * Halo: Silentium