Chambers of Repose and Waking

The Chamber of Repose (眠りの部屋) is a room in Hollow Bastion built by the apprentices of Ansem the Wise. It has a sister room called the Chamber of Waking (目覚めの部屋). It is a mysterious room that Xemnas was looking for in Kingdom Hearts II but it has never been shown and was only talked about in reference to the Chamber of Repose.

Chamber of Repose
After the apprentices banished Ansem and became Nobodies, Xemnas would travel to the room. In the scene in Kingdom Hearts II Final Mix, Xemnas entered Ansem's computer room, entered a data disk into the machine, and entered the password "Another", after which he entered six more passwords: the original names of the six apprentices of Ansem the Wise.

This caused a passage to appear below the Heartless Manufactory, which had a long path spiraling down to a long hall with doors locked with electronic locks and bound with chains. At the end of the hall was a door. Inside was a single throne which Xemnas would sit on, as well as the armor of a Keyblade warrior named Aqua, including the Keyblade. 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. He also told Zexion he suspected that there was a link between the Chamber of Repose and Castle Oblivion, where, allegedly, the Chamber of Waking is.

Chamber of Waking
It is said that the Chamber of Waking may contain his other "friend" who is suspected to be Ven. This room is also said to be 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 Riku left the castle with DiZ and King Mickey, and Sora was put to sleep.