Requiem

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, unofficially named the "legendary planet", is a mysterious world toward which the aft section of the, containing John-117 and Cortana, is seen drifting towards in the Legendary epilogue of the game Halo 3. Seconds later, a light shines across the surface of the planet, as the Forward Unto Dawn is caught by the planet's gravity. The same fate happened to. The planet also appears in a small image in the Halo Encyclopedia, where it's shown to have a reddish-orange color on a small illuminated section. This is the location seen in the Debut Trailer for Halo 4.

It has been confirmed by 343 Industries in the Making Halo 4: First Look video that it's in fact a Micro Dyson Sphere‎, this is in conjunction with the information that the Halo 4 multiplayer map Haven is a solar generation facility used to create light for the local life forms. "Instead of surrounding a star, the massive exterior shell of Requiem encases a planet - a variation on a Forerunner Shield World."

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.

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.

Shield World
Given the planet's clearly-artificial appearance, it's possible that it is one of the many Shield Worlds constructed by the Forerunners as safe havens to protect them from the Flood and the activation of the Halo Array.

One theory suggests that the common Forerunner symbol seen on the surface solely represents a Shield World, relying on the fact that the cover of Ghosts of Onyx bears this symbol and the book prominently features a Forerunner Shield World. Also, all Shield Worlds seen so far are disguised as natural planets, whereas the Legendary Planet bears clear evidence of artificial Forerunner construction.

Another point supporting the theory is that there appear to be cities on the surface.

Reading the Terminals, an interesting phrase appears on Legendary, that 05-032 Mendicant Bias, a Forerunner AI who defected to the Gravemind and was later defeated by Offensive Bias, says directly to Chief, stating:


 * "Atonement.
 * And so here at the end of my life, I do once again betray a former master. The path ahead is fraught with peril. But I will do all I can to keep it stable - keep you safe. I'm not so foolish to think this will absolve me of my sins. One life hardly balances billions.
 * But I would have my masters know that I have changed. 
 * And you shall be my example." - Mendicant Bias

The last line has caused fans to believe that Mendicant Bias had an intention of showing the Forerunners an example of his "atonement" who is the Master Chief. Knowing this, many believe that the AI protected the Master Chief and prevented a number of unseen events on the Ark and Installation 04B from affecting the Chief and his missions. Some have also theorized that he deviated the portal's course at the last moment before he was destroyed. Though no information supports this and it has been neither stated nor implied, it can be supported logically in that the Ark's Portal might have several possible entrance/exit points throughout space; the Forerunners adapted to the Flood with extensive redundancy in all systems and escape mechanisms, so it is possible that the Forerunners on the Ark established themselves at a different Portal destination, possibly one of many, and Mendicant Bias shifted to this destination when the Forward Unto Dawn was halfway through, thus severing the ship and dropping the Master Chief's half near the colony.

The Domain
A new possibility has been revealed by the Halo Anniversary terminal videos. Frank O'Connor of 343 Industries indicated that these videos would release hints about the upcoming Halo 4. In the 2nd terminal video, Guilty Spark is discussing the final deployment of the Halo array, as well as the possibility of the loss of everything that the Forerunners knew. At the moment of his discussion with Installation 03's Monitor the legendary planet is specifically and prominently shown as Guilty Spark mentions "All the knowledge of the Forerunners", suggesting that perhaps the world contains the source of the domain collective knowledge.

Location
According to Cortana in the epilogue the Forward Unto Dawn was travelling through the portal entrance as it collapsed, but that only "some of us [made it through]" - the Arbiter in the fore half. Based on this it would appear that the planet is located somewhere outside of the galaxy not far from the location of Installation 00, and made more evident by Cortana's mention of the Ark's status - heavily damaged, as opposed to "destroyed" as 343 Guilty Spark predicted.

Trivia

 * If you look very closely you can see strange marks on the planet that resemble Forerunner technology.
 * John-117's fate parallels King Arthur's. He sleeps (in a cryotube) on a mysterious island (in an unknown part of the Galaxy, or on the Forward Unto Dawn), until the day when he is needed. ("Wake me when you need me.")
 * In previous images, it seems as though the symbols seen on the planet suggest the planet is made up of machinery. But as seen in the early Halo 4 concept art, the supposed Legendary Planet is shown to have organic life, such as land and water, similar to the Halos', both being Forerunner.
 * If what Mendicant Bias says is true, then it's speculated that the Flood or perhaps a more dangerous threat is still present, possibly the Precursors.
 * In Halo: Combat Evolved Anniversary level The Pillar of Autumn, on the notice board on the bridge, there is a picture of what appears to be the Unidentified Planet with a "Trollface" imprinted in the bottom left hand corner.
 * 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". This could be possible hint to the background of the planet. The name could imply that it is a storage area (mass) for the Flood (the dead re-animated), or that it is the home of a forgotten civilization (Forerunners or Precursors) that was thought to be dead.
 * It is hinted that there might be living Forerunners on Requiem.
 * Many Huragok know that requiem is home to the didact, however they do not know its location.

List of appearances

 * Halo 3
 * Halo 4