The Fire Temple, also called the "Demon Temple" or "Daemon Temple" or "Temple of the Abyss," is an overland Monster spawn. It is located on Avatar Isle, also called Fire Isle (or Avatar Island or Fire Island). (This is the same island with the Dungeon Hythloth and the Shrine of Humility.)
The temple's central structure consists of two "levels," basically a large room and a flat area on top of that room with stairs leading up to it. The flat area is extremely disturbing, with blood and decaying human body parts strewn about the place.
The implication, that this is a place of great evil and vile rituals used to take place there (or perhaps still do), is clear.