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 shined across the surface of the planet, as the FUD 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. While many believe that it is Onyx, 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 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. 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.

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. The Marathon and Halo universes' contradict each other in a number of ways. For example, 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. Another aspect in which Marathon clashes with the Halo trilogy, is that the Pfhor in the Marathon are the first ever alien species encountered by humanity, but the Covenant are encountered before when Marathon takes place.

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. Another point supporting the theory is that there appear to be cities 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 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. It is also believed that he deviated the portals 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 Star Control II "Rainbow World." - This would explain the references to the Precursors in terminals and other supplemental material throughout the game, and explain why Didact wished to "Follow in their footsteps," as the Precursors made the Rainbow Worlds of Star Control to point the direction in which they migrated to.

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 to 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. 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.

Onyx
Many fans support the idea that the planet is Onyx but it is highly unlikely. The planet deassembled into the trillions of Sentinels that made it up. But 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.

Trivia

 * The back of Forward Unto Dawn crashing on the planet would be another reference to Marathon.