Thieves Hideout is the fourth dungeon area in the game, located in Emerald City. It becomes accessible at levels 16-20, and the mutated version at levels 66-70. It is preceded by the Forest, also in Emerald City, and followed by the Beach in Oceana.
The area boss for this dungeon is the Thief Leader, and in the mutated version you may encounter the Demon Leader possessed boss.
The Priestess ally can be found in this area.