Chambers of Repose and Waking
The Chamber of Repose and the Chamber of Waking are mysterious rooms that seem to hold some great importance to Xemnas, due to their connections to his past. Xemnas often sits in the Chamber of Repose in order to regain memories of his past as Terra and Master Xehanort, and seeks the Chamber of Waking in Kingdom Hearts 358/2 Days and Kingdom Hearts II Final Mix.
Kingdom Hearts Birth by Sleep
Following Ventus entering a coma-like state, Aqua transforms the Land of Departure into Castle Oblivion. The throne room of the Land of Departure becomes a chamber that looks very similar to the Chamber of Repose, only instead of Nobody sigils, the room is adorned with Terra's Mark, and the room is noticeably brighter. There is a single, tall throne in the center, where Aqua lays the comatose Ventus.
Between Kingdom Hearts Birth by Sleep & Kingdom Hearts 358/2 Days
After the apprentices banished Ansem the Wise, but before they became Nobodies, the apprentices created the Chamber of Repose within the Radiant Garden's castle. Xemnas would frequently travel to the room, as if something within it reminded him of his forgotten past. Before Xemnas entered the chamber, he goes to Ansem's computer room, inserts a data disk into the machine, and typed in the password "Another", after which he typed in six more passwords: the original names of the six apprentices.
This caused a passage to appear below the Heartless Manufactory, which had a long stairway spiraling down to a long hall. Inside this hall were doors electronically locked and bound with chains. At the end of the hall was a room containing a throne for Xemnas to sit on, as well as the armor and Keyblade of a warrior named Aqua. Xemnas would speak to the armor, addressing it as a "friend".
Apparently, Xigbar often eavesdropped on the conversations, and told Zexion that he heard another voice once. Having been there since the beginning and knowing Terra has two close friends, he correctly speculated the Chamber of Waking would contain his other "friend".
Kingdom Hearts 358/2 Days
The Chamber of Waking contains Xemnas's other "friend," who is Ventus. This room is found in Castle Oblivion. Whether or not Xemnas ever found this room is unknown, although based on conversations between Saïx and Axel in Kingdom Hearts 358/2 Days, it is unlikely that Xemnas has, since Axel was ordered to search for the room after he had disposed of the Organization's traitors, but failed to find such a room.
Kingdom Hearts 3D: Dream Drop Distance
Kingdom Hearts Re:coded
A Fragmentary Passage
Ventus is seen mysteriously vanishing from the chamber.