Requiem

At the end of cutscene for the Legendary ending of Halo 3, The section of the UNSC Frigate Forward Unto Dawn containing Master Chief and Cortana, is seen drifting towards an unknown planet, when light is shined across the surface of the planet there is a large Marathon symbol on the surface. Many believe this to be a Forerunner planet that will be the next destination for Master Chief.

Possible Theories

 * This is Onyx, and Master Chief will reunite with Dr. Hasley, CPO Mendez, and the remaining Spartan II's and III's. Due to it's intact shape, it would likely be Onyx before it disassembled itself.
 * It is the planet that the game Marathon took place on.