Bloodstained: Ritual of the Night/Celeste's Room

To enter Celeste's Room, you first need to acquire Celeste's Key. Starting from the warp room in the Hall of Termination, go to the top of the next room. The center of the ceiling is breakable and reveals a hidden room. The middle chest contains the key.

Celeste's Room is located near the top save room in Livre Ex Machina. Inside are several music boxes that you can open and close by pressing. There are also two chairs and if you sit in one of them, a ghost will materialize sitting on the other.