Requiem

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Requiem, also known as Shield 0001 and unofficially named the "legendary planet", was a Micro Dyson Sphere‎ in which the Ur-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.

History
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.

Layout and Function
Requiem was a massive Forerunner Shield World that was used as a military base and research facility. It consisted of a large metal outer shell with several large entrance apertures. Within this shell was another planetoid. This planetoid had a terraformed surface that was habitable. The planetoid itself was also hollow and had a surface on it's interior similar to Shield 0459. At the exact center of this planetoid was a docking station where the Didact's Cryptum was located. Although the interior of the planetoid was sparsely settled and consisted of barren rocky terrain, the exterior of the planetoid was covered in structures of various design and function as well as numerous different environments that ranged from lush jungles to snowy tundras and from mountainous areas with cave systems to vast seas dotted with islands. Forerunner structures could be seen looming in the distance from nearly every location on the planetoid, whether they were hanging from the metal ceiling that was the outer shell, rising from the ground to great heights, or floating in the air somewhere in between.

First Battle of Requiem
The Didact along with an army of Promethean Knights that were sealed away inside of Requiem 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 supposedly be able to communicate with the Infinity clearly, only to see the interior of the Cryptum open up as the Didact appeared from within and assembled body armor around himself before standing up to see Covenant Remnant forces kneeling before him as he waved his hand; causing the Prometheans to come under his command, before levitating John-117 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 that once belonged to him. Despite the efforts of John-117 and Cortana (mostly due to Cortana's growing rampancy), the Didact succeeds in leaving with a Covenant fleet to Installation 03 followed by John-117 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 in 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.
 * It is later seen in the Halo 4 terminals that Requiem has more than one entrance portal.
 * "Requiem" comes from the Latin requiem and means rest (mostly after death).
 * A requiem mass is a musical mass written to celebrate or commemorate the death of an individual. The title of the last movement of such a mass is also Requiem.
 * 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.
 * In Halo: Combat Evolved Anniversary, in the level The Pillar of Autumn, on the bridge's bulletin board, there is a sign that features Requiem along with a Troll Face, which was 343 Industries "trolling" the player about Halo 4 (note that you must be playing with the HD graphics on to see this sign).
 * 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 either the shell moves and the planet itself doesn't, or Requiem rotates freely with the 'sunlets' actually changing position in the ceiling.
 * While on board the UNSC Forward Unto Dawn in Halo 4, a hologram of Requiem identified it as 43121124.
 * 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 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.
 * Requiem consists of three layers. The outermost layer is the mechanical exterior. The inside of this layer becomes  the "sky" for the second layer which appears as a planet within a planet. the third layer is the interior of the second layer. This third layer is similar to the structure of the Shield World in Halo Wars as it has the ground in the inside surface where the orientation of "up" would be towards the center of the sphere.

Appearances

 * Halo 3
 * The Commissioning
 * Scanned
 * Halo 4
 * Spartan Ops
 * Halo: The Thursday War
 * Halo: Silentium
 * Halo: Escalation