Requiem

The unofficially dubbed "Legendary Planet" is an unknown planet toward which the rear section of the UNSC Frigate Forward Unto Dawn, containing Master Chief and Cortana, is seen drifting to in the Legendary ending of Halo 3. Seconds later, a light shined across the surface of the planet. 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
There are many speculations on what this unknown planet might be. Some people believe it is Onyx, although it does not seem to match the description of the mechanical planet found in Ghosts of Onyx, making this somewhat unlikely, plus the fact that Kurt set off nuclear warheads in the planets 'core' and likely led to the destruction of Onyx. Also the fact that Onyx was found out to be trillions of Sentinels put together. The assumption of this is a planet being a Forerunner Shield World, could be possible. The Ark portal would have to have a fail-safe location to transport to. Some also believe that the terminals in the game are a reference to the unknown planet. However, it is clearly inhabited by some advanced species, or was at one time, and by the Forerunner symbols, they are the natural alternative of inhibitors, at least 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 universes. 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 home world. 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 appears to be oceans and cities on the surface, similar to those seen numerous places on the Ark, excluding the theory of Sangheilios, and most likely the Shield World-theory as well.

Forerunner Homeworld
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 his masters (the Forerunners) an example of his "atonement" who is the Master Chief. Knowing this many believe that the AI, protected 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 nanosecond before he was destroyed.

Harvest
Another possibility is the planet Harvest, which Halo Wars takes place on. The basis for this is the patterns on the unknown planet are slightly similar to the ones on Harvest shown in some of Halo War's Trailers.

Harvest is known to have large Forerunner structures, which is why it was such a big importance in the Human - Covenant War. Whatever the Forerunners used Harvest for would most likely have a great affect on the Halo Universe. It would make sense for Master Chief to land in such an important place.

"Rainbow World"
One final 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. Others think 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 very probable, as the Forerunners would have most likely designed a safe haven area after a Halo Activation.

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.

Flood World
It is possible that this could be the orignal Flood planet.If true the Forrunners probably went through the portal when it closed on accendent or to see what would happen, and then they found a planet that carried other life forms that were being consumed by some creature, they probably had some people on board get it infected by flood and brought back somehow.

Trivia

 * One of the AIs in Marathon named Tycho had crashed into one of the Lh'owon moons, as stated by the U.E.S.C. Marathon A.I. Durandal. The back of Forward Unto Dawn crashing on the planet would be another reference to Marathon.