Originally, Halaster Blackcloak was not native to the Forgotten Realms. He hailed from another plane of existence and arrived in the Forgotten Realms thousands of years ago, where he began constructing Undermountain as a place to conduct his arcane experiments.
Over time, the dungeon grew beyond even his control, becoming a sprawling labyrinth filled with monsters, traps, and treasures, attracting adventurers and fortune-seekers from across the realms.