The Golem Boss is found at the end of the Dwarf tunnels located in randomly spawned volcanoes . He waits in a room filled with bookshelves and 8 large chests. The entrance to the room is large enough for a player to fit through, but too small for the Golem to, making it quite easy to shoot at him with arrows. At near 20% health left, the Golem will go into a Beserk mode, turning red and presumably doing more damage.
The main loot obtainable by the Golem Boss is the Dwarven Spear and experience, the 8 chests contain mostly random mid to high level loot, favoring metals and music discs.