Fortress at World's End
The Fortress at World's End is a name given to an ancient fortress built into the side of the cliffs at the northern edge of the Frostland. The fortress was carved into the rock, and the walls covered with magical ice. Though the fortress's original inhabitants are long gone, it was recently occupied by Tephra, who used it as a base of operations for herself and her minions until she was defeated at the hands of the Challengers of the Unspeakable.
The fortress appears to be several thousand years old, dating back to before the Elf Wars, and one room housed dozens of coffins containing deceased derro. Whether they built the fortress or merely used it for a time is unknown.