Requiem



This unidentified planet, unofficially named the "legendary planet", is a mysterious world toward which the aft section of the frigate, 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 planet also appears in a small image in the Halo Encyclopedia, where it is shown to have a reddish-orange color on a small illuminated section. This may be the location seen in the Debut Trailer for Halo 4.

Background
The planet is littered with blue-glowing symbols and geometric patterns - clearly Forerunner in origin. At least two of the symbols are clearly recognizable. 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 can also be observed on the surface. It is known to be a common Forerunner symbol, used in multiple Forerunner constructs, and also used to refer to a Reclaimer. The Seventh Column can be seen on the Legendary Planet as well.

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 an elegant story device meant to keep the series open to future game titles or novels, and leaves the opportunity for the Master Chief and Cortana to continue their adventures well into the future.

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, which leads to some believing that the planet is the planet that Catherine Elizabeth Halsey, Franklin Mendez, Blue Team and the remaining SPARTAN-IIIs and SPARTAN-IIs were transported to. However, the common symbol on the cover may simply reference to the fact that the story of the novel puts strong emphasis to the Forerunners. Furthermore, if the symbol only represented a Shield World, its common appearances on the Halo installations and the Ark would be unexplained. Also, all Shield Worlds seen so far are disguised as natural planets, whereas the Legendary Planet bears clear evidence of artificial Forerunner construction. Also ,however, in Halo: Reach, Jun is transporting Catherine Halsey, and could not have been their in the events of Ghosts of Onyx, thus sparking debate over which is canon: novels or Reach's timeline.

Forerunner colony
Yet another theory is that it could be an abandoned Forerunner colony or planetoid-class ship, similar to High Charity. Throughout the Halo franchise, there have been a few mentions of the Forerunner colonies prior to the discovery and subsequent war with the Flood, notably in the Halo 3 Terminals. This theory provides an alternative to the Shield World theory, as well as explaining the planetoid, the Forerunner symbols, and providing an opening for sequels; however, if the Forerunners had indeed been living on this planet-like object, they would have been killed when the Halo array was fired (unless, like the Ark, the planet is not located in the Milky Way Galaxy, placing it outside of the effective range of the Halo Array). It may also be infested with flood due to the flood infecting so many planets and that may be why it was abandoned.

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.

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

 * John-117's fate parallels King Arthur's. He sleeps (in a cryotube) on a mysterious island (planet), until the day when he is needed. ("Wake me when you need me.")

List of appearances

 * Halo 3 ( Legendary epilogue )
 * Halo 4 ( Thus far, only in the teaser-trailer and teaser-posters. )