Some time after the Third Overlord disappeared, before the Cataclysm destroyed the Dark Tower, Gnarl and the minions fled into the Netherworld to wait for a new Master. When the Dark Tower was destroyed, the minions took residence there permanently. When the Overlord first arrives, the Netherworld Tower is only a mess of rock caverns but progressing trough the game will make it cleaner and will become a true palace worthy of a Overlord