Requiem

The unofficially dubbed "Legendary Planet" is a mysterious planet toward which the rear section of the UNSC Frigate Forward Unto Dawn, containing the Master Chief and Cortana, is seen drifting in 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. At Comic-Con 2010, Frank O'Connor hinted that future media featuring the Master Chief after the events of Halo 3 may be produced, meaning that more information on the planet may be revealed in the future.

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, and on the cover of Halo: Ghosts of Onyx. There are 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 constructs, and also used to refer to a Reclaimer. The Seventh Column can be seen on the Legendary Planet 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 meant 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.