From Halopedia, the Halo wiki
The Altar is a structure on Forerunner Halo Installation 03. In 2557, Jul 'Mdama's reformed Covenant and his Promethean allies attacked Gamma Halo and retrieved the Conduit. They brought the device to the Altar, where they intended to insert the Conduit into a machine located in the center of the complex in order to open slipspace portals for Prometheans to invade and give the Covenant control of the installation.
List of appearances