Halaster Blackcloak

Halaster Blackcloak was the infamous mad archmage of undermountain.

Long ago before Waterdeep existed, he arrived with his 7 apprentices to make his lair in an old abandonned dwarf fortress. Where he came from is unknown. A wizard of undeniable prowess, he soon transformed the ancient caves into a fearsome dungeon, in fact the rumored deadliest and certainly most renownned of all Faerûn by "collecting" a huge number of extraplanar monsters and installing numerous traps and portals.

Using his magical talents, he turned Undermountain into a quasi-mythal, gaining powerful abilities and immortality within his complex. The drawback being that this turned him also completely mad while he was in his dungeon.

Expedition to Undermountain states that Halaster died trying to prevent an unknown magical threat dwelling (or fated to dwell) in undermountain. Remnants of his spirit haunt his old lair, not a ghost but rather memories.