Requiem



The unofficially dubbed "Legendary Planet" is a mysterious planet toward which the rear section of the UNSC Frigate Forward Unto Dawn, containing 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.

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, as well as on the cover of Ghosts of Onyx. There is 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 contructs, and also used to refer to a Reclaimer. A pattern which can be interpreted as the number seven can be seen on the surface 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 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 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. 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.

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 object, they would have been killed when the Halo array was fired. Another point supporting the theory is that there appear to be cities and oceans on the surface, similar to those seen numerous places on the Ark. The appearance of these could also apply to this "planet" being out of range of the "Halo effect", which could mean there are Forerunners alive. This could also mean that these Forerunners could have been out of range of the Flood and therefore not contacted by their people in the hopes of "rebuilding the empire from scratch".

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 04 (II) 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.

Onyx
Many fans support the idea that the planet is Onyx, although there are a number of discrepancies. It does not seem to match the description of the mechanical planet found in Halo: Ghosts of Onyx; in addition to the fact that Kurt-051 set off nuclear warheads in the planet’s core and destroyed Onyx, leaving only a small slipspace portal guarded by trillions of Onyx Sentinels that would fire on anything remotely close to the core, makes this theory unlikely. However, supporters of this theory specualte that it would not take the sentinels a very long period of time to rebuild the planet. Considering how quickly The Ark was able to complete a second Installation 04, it is likey that the trillions of sentinels guarding it may be very effective at constructing a new planetary surface. However, considering the fact that these machines were never programmed to build anything, this theory still remains highly unlikley.

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, this is inaccurate – the “portal” is suspected to instead be the Iris Nebula, 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 to question how Cortana knew that the firing "did a number on the Ark", while the first theory brings with it the question of how the Master Chief survived the Halo's firing sequence. Others believe that the Frigate managed to get halfway through the portal, and was dumped out at a random destination, or even a planned fail-safe area. This theory is possible, 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.