The Sarcophagus, officially called Shield World 00 6 and previously hidden within slipspace surrounded by the planet Onyx, is a Forerunner shield world designed to act as a refuge for the Forerunners during the Forerunner-Flood war in the Zeta Doradus system. The world was never populated by Forerunners, however, primarily due to politics that resulted in the extinction of Forerunner society. The Sarcophagus, while physically occupying a 23-centimeter sphere within the planet Onyx, is internally the size of the Earth's orbit around the sun, containing a surface area approximately 500 million times larger than that of Earth's.
During the Onyx Conflict in 2552, UNSC forces gained access to the interior of the Sarcophagus, eventually bringing the entire facility into real-space. This led to the establishment of a research facility on the interior surface called ONIRF Trevelyan, which was used to not only study the shield world, but acted as a hidden facility for ONI to perform research away from the eyes of the public. After the activation of the Guardians by Cortana in 2558, the facility was placed back into its slipspace bubble, leaving no visible marker that the shield world even exists.