Requiem

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Requiem, unofficially named the "legendary planet", was a Micro Dyson Sphere‎ in which the Didact was imprisoned inside by the Librarian. Instead of surrounding a star, 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 after the first level of Halo 4.

Background
The planet's surface is littered with blue-glowing symbols and geometric patterns - clearly Forerunner in origin with at least two clearly recognizable symbols. The surface of the planet bears a certain Forerunner symbol that has appeared in wall ornamentation on all of the Halo installations, on the surface of the Ark, and on the cover of Halo: Ghosts of Onyx. There are a number of theories concerning the meaning of this symbol, some of which interpret it as the main emblem of the Forerunners, the symbol for the Halo Array and the related installations, or possibly a Shield World.

A Marathon symbol, a common Forerunner symbol often used to refer to a Reclaimer and The Seventh Column, are all seen on the surface of the Legendary Planet.

Speculation on just what the planet is, its purpose, and even its location are varied. Whatever the case, it is clearly inhabited by something, or was at one time, and by the Forerunner symbols, it seems to have at least been occupied by the Forerunners in the past. Although its purpose and identity are obscure, the planet is a story device meant to keep the series open to future game titles or novels and leaves the opportunity for John-117 and Cortana to continue their adventures well into the future. A video that confirmed the main menu of Halo 4 shows the remains of drifting towards Requiem, which appears to be damaged, with a Halo in the background.

As revealed by the Terminals, Requiem was the Didact's base of operations during both the Human-Forerunner and the Forerunner-Flood wars. 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.

In the Halo novel Halo: The Thursday War it was revealed by Prone To Drift that Requiem may be the resting place of The Didact. There are no portals leading to Requiem for the safety of the Didact. This was confirmed in Halo 4, Requiem is a Shield World that serves as a prison for The Didact by The Librarian and a small group of Sentinels that serve as her caretakers. 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 Storm Covenant 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 Storm Covenant 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.

Six months later, the Covenant maintains a presence a Requiem. The UNSC dispatched Infinity to clear out the Covenant and Promethean drones in order to setup research bases. Requiem was eventually destroyed when Jul 'Mdama sent the planet into the sun, causing a supernova with intent of destroying the Infinity along with it.

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 one of the final pieces written by the famous composer 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.

List of appearances

 * Halo 3
 * Halo 4
 * Halo: The Thursday War