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. If a user freezes the cut-scene, they can see that there is a large Marathon symbol on the surface of the planet. The symbol has lead to much speculation to whether it is Tau Ceti IV or just another in a long line of Marathon references.

Background
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.

Onyx
Many fans support the idea that the planet is Onyx but it is highly unlikely.

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.

It may be possible that when the threats were gone (The humans and Covenant), that the Sentinels rejoined to form the planet surrounding the Slipspace field, but this is just pure speculation.

It may also be possible that the Shield World was sent to the Ark's location after the battle. Had the Shield World exited slipspace completely, it would show its actual size to the outside.

Marathon
It was postulated early on that this planet was Tau Ceti IV and that the cyborg portrayed in the video game Marathon is actually the future Master Chief. This theory coincided well with the concept of the "Eternal Hero" Bungie is famous for, however the resemblance seems to be only a nod to fans.

Bungie has confirmed the Halo series and Marathon Series do not take place in the same universe. In addition, the Marathon and Halo universes contradict each other in a number of ways.


 * The UESC Marathon left Sol in 2472 and arrived at Tau Ceti in 2773, but by 2552, in the Halo Universe, the UNSC already had a presence in the Tau Ceti system and a large number of other worlds, while in Marathon, it was the first planet to be colonized by Humans.


 * The protagonist of the Marathon series had been in cryostasis on board the UESC Marathon for the last 300 years, from 2472 to 2773, making it impossible for him to be John-117. In addition, the game's manual reveals that he was born on Mars, while the Master Chief was born on Eridanus II, hundreds of years later.


 * Yet another aspect in which Marathon clashes with the Halo trilogy is that the Pfhor in the Marathon Universe are the first ever alien species encountered by humanity, while the Covenant in the Halo Universe are encountered before Marathon takes place chronologically.
 * It is also worth noting that the Pfhor in the Marathon Universe are primitive, while the Humans in Marathon are technologically advanced. While in the Halo Universe The Covenant are technologically advanced and the Humans were primitive.
 * It should also be noted that the humans of the Marathon series have not yet invented faster than light travel while the humans of the Halo universe have.

Shield World
Given the Legendary planet's clearly artificial appearance, it could be one of the multiple Shield Worlds constructed by the Forerunners as safe havens to protect them from the Flood and the activation of the Halo Array.

Forerunner Colony
Yet another theory is that it could be a 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 doing so for nearly one hundred thousand years. It is reasonable to assume that after such an extent of time, the Forerunners would likely have once again risen to power to become an influence throughout the Galaxy, bringing to question the validity of this particular theory. 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.

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.

"Rainbow World"
Another theory deals with the legendary planet being similar to a Rainbow World from Star Control II. The Precursor race in Star Control series made the Rainbow Worlds to point the direction in which they migrated to. This would explain the references to the Precursors in the Terminals and other supplemental material throughout the game, and explain why Didact wished to "Follow in their footsteps."

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.

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.