Sanctum of the Hierarchs

The Sanctum of the Hierarchs is the name given to the private quarters of the High Prophets on High Charity. They are restricted to virtually all other creatures, except the Honor Guard, Prophet and Elite Councilors and high ranking Covenant officers, such as Rtas' Vadum and The Arbiter.

Description
The Inner Sanctum featured unique architecture, highly ornamented doors and various other luxuries. It is also known that the inner walls of the Sanctum were glazed with smooth glass from all the worlds conquered by the Covenant, much like the Step of Silence. The Sanctum has several entrances; one is via a lift in the Council Chamber, and another is a large hallway where dozens of Honor Guards used to stand watch. The higher levels of the Sanctum, or the Far Tower, also include three tenders for Phantom dropships.

Many events took place here, including the "exchange of hats" when the Brutes supplanted the Elites as the Honor Guards and minders of the Prophets, after the assassination of the Prophet of Regret. It was here that the Prophet of Truth and the Prophet of Mercy explained to the Arbiter his task of landing on the surface of Installation 05 and retrieving the Index from the Flood infested Library, which took place in two levels through the Sentinel Wall and the Quarantine Zone.

In the end, the Flood used Pelican dropships to land in the Sanctum. It was one of the first areas to be infested; it was filled with spores and Flood "structures" when the Master Chief traversed the area in search of the High Prophet of Truth.

The room that Cortana was located in during Halo 3 was the Sanctum. The Sanctum was connected to the reactor room that was not accessible in Halo 2 due to a series of locked doors. The room had been dramatically altered by the Flood infestation: there were widespread structural failures that occurred as a result of High Charity colliding with the surface of Installation 00.

Trivia

 * In Halo 2 when you enter the room, you can see a dead Elite Minor with an Energy Sword on the nearby obelisk and another on a pronged pedestal.
 * The Sanctum of the Hierarchs is apparently Gravemind's "control" room.