The Temple of the Fiends is an ancient fortress situated to the northwest of the city of Coneria. It is said to be the site of a cataclysmic battle 2,000 years ago. The Fiends reappeared about 1600 years later at the Temple before dispersing into their new strongholds throughout the world.
Garland used the Temple to hold Princess Sara prisoner until the Light Warriors freed her. The Temple is now inhabited by various monsters, most noticeably an army of undead warriors known as Bones. Two of the doors in the Temple are locked and can only be opened by the Mystic Key.
Final Fantasy Origins: Temple of Chaos