Castlevania: Order of Ecclesia/Large Cavern

The large cavern is an optional area that can be accessed from Dracula's castle. It contains a series of rooms that lead to the boss. Each room can be cleared either by defeating all enemies or after a set amount of time has elapsed. Between every two rooms is a vertical room with no enemies that contains chests.

Boss: Jiang Shi
You can absorb the Fidelus Mortus glyph from Jiang Shi.

Once defeated, Jiang Shi will be sealed. You can attack the seal to release him and fight him again. This can be repeated indefinitely making this area an effective place to level up. Defeating Jiang Shi also satisfies Irina's third quest, An Unwelcome Guest.