Requiem

The unofficially dubbed "Legendary Planet" is a mysterious planet toward which the rear section of the UNSC Frigate Forward Unto Dawn, containing the Master Chief and Cortana, is seen drifting into the Legendary ending of 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 Legendary 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.

Background
The planetoid is littered with blue-glowing symbols and geometric patterns - clearly Forerunner in origin. At least two of the symbols are clearly recognizable. This could mean that the planet is the Forerunner homeworld, because the planet that is shown in the Halo Legends episode, Origins, appears to be the Forerunner homeworld and bears similar symbols on its surface.

The surface of the planetoid bears a certain Forerunner symbol that has appeared in wall ornamentations 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 Legendary 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.

It's probable that the next Halo game will be focused, at least in part, on this unidentified planet.

Shield World
Given the Legendary planet's clearly artificial appearance, it could be 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.

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).

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 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.

Forerunner homeworld
Another possible identity is that the planet is the Forerunner homeworld, since the Forerunners would doubtlessly have wanted one of their ships to get to the homeworld safely if something similar happened to a Forerunner vessel as did Cortana and the Chief.

It strongly suggested in the Halo Legends episode "Origins" that the Unidentified Forerunner Planet, a Forerunner colony, is indeed the Legendary Planet. Hints include what looks like the Forward Unto Dawn drifting towards the planet, and Installation 00 orbiting a rocky, brown moon, which in turn is orbiting the planet in question.

Location
Even the location of the planet is a mystery. Some believe that it is located relatively close to the Ark, basing this theory off of the nebulous clouds seen around and behind the Ark and the portal also being seen multiple times in the ending sequence. However, some suspect the “portal” to be the Iris Nebula instead, located well within the galaxy, though extremely distant from UNSC or Covenant territory. Normally, the space around and between galaxies is devoid of almost all matter, with Installation 00 being the sole known exception. Whether the vessel was left relatively close to the Ark, or the back half of the frigate managed to fit into the portal and get thrown out at a random destination is unknown.

The latter theory brings into question how Cortana knew that the firing "did a number on the Ark"; however, Cortana was present to witness 343 Guilty Spark's claim that the premature activation of Installation 04B would destroy the Ark, explaining this knowledge. The first theory, however, brings with it the question of how the Master Chief survived the Halo's firing sequence. Some have noted that the ring was incomplete when it was fired, yet it has been confirmed that the activation worked as it was intended to and completely eradicated the Flood infestation; this effectively renders the theory that the planet is adjacent to the Ark impossible, as there is no possible way that the Master Chief could have survived the ring's activation if he had been close by.

Others believe that when the frigate managed to get halfway through the portal, it was not simply dumped out at a random location, but a planned fail-safe area. This theory is possible - even likely, given the incredibly low probability that, out of every other location in the entire galaxy that the Forward Unto Dawn could have possibly been sent (assuming that the Legendary Planet is even located within the Milky Way Galaxy), the aft section of the frigate would simply happen to end up directly next to a prominent Forerunner construct - as the Forerunners may have designed a secondary safe haven to be used after a Halo activation (the primary location being the Ark). The planet may have been a safe haven in case the Ark was contaminated, destroyed, or unreachable. As mentioned above, it is also possible that the fail-safe location is the Forerunner homeworld itself.