The Secret Altar (Japanese: 秘密の祭壇 Secret Altar) is the final room in Luigi's Mansion. It is found in Area Four and is where the game's final boss, King Boo, resides. After Luigi retrieves the spade key in The Artist's Studio, he accesses the Secret Altar and defeats King Boo.
The Secret Altar contains three chandeliers, one of which holds a Blue Stone. On the west wall is a stone lion's head on the other side of which is the Bottom of the Well (which is where the player first sees the room). On the east wall is a mirror flanked by two torches burning with incense. If Luigi vacuums the incense, one torch gives him money, and one torch gives him a gold bar. Two similar torches are found at the rear of the room on either side of Mario's painting. These and the two chandeliers near King Boo can be vacuumed, but they do not yield anything.