The Golden Wastes
The Faceless One was the master of the labyrinth of chaos underneath Velos. There he conducted obscene experiments in a place sacred to Chaos. He colluded with the White Flame Brotherhood and Mountain Saurians to create a fortress unknown to the Overlord and his servants.
The PCs discovered clues to his existence and in a several month search located his lair and destroyed him. However, after they did so they discovered the mysterious Faceless One to a half-real Simulacrum. They seized what treasure they could, including his notes, and went back to the surface to discuss their next moves.