Requiem
From Halopedia, the Halo wiki
In the Legendary ending of Halo 3, the rear section of the UNSC Frigate Forward Unto Dawn containing Master Chief and Cortana, is seen drifting towards an unknown planet. Seconds later, a light shined across the surface of the planet. If a user freezes the cutscene, they can see that there is a large Marathon symbol on the surface of the planet.
Speculation
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. The assumption of the is planet being a Forerunner Shield World, could be plausible. The Ark portal would have to have a failsafe location to transport to. Some also believe that the terminals in the game are a refference to the unknown planet.
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.
Although it's 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.
