The Core Room Antechamber was a massive chamber located at the center and core of the Forerunner-constructed Onyx. The Antechamber served as the only entrance to the Onyx Shield World. Upon the powering up of the Halos, the Antechamber is always reachable by and via specific Slipstream Translocation Devices, however, the Core Room is only accessible for a short time and duration after the Halos are activated.
The actual entrance to the Shield World was itself in the center of a large stepped structure within the Antechamber, which Franklin Mendez compared to Hell in Dante's Inferno, although it was really more representative of Mount Purgatory. The structure would gradually 'flatten' until it became level with the ground. At this point, the entrance to the Shield World would be sealed.
The Antechamber was the site of a desperate rearguard and strategic action by Spartan-II's and Spartan-III's led by Spartan-051 against Covenant forces under Voro 'Mantakree during the Battle of Onyx in 2552. The Antechamber was destroyed with FENRIS Nuclear Warheads by Spartan-051, who sacrificed himself to deny the Covenant access to the Shield World.