Halaster Blackcloak

Halaster, the Mad Mage of Undermountain, is the deranged individual behind most of the traps and horrors found in the great dungeon under Waterdeep. Undermountain is his home, an amusement gallery in which others perform to entertain him.
The Mad Mage knows the ever-changing ways of Undermountain as no one else does, for he is the one who controls those changes. He prefers to remain unseen, skulking about invisibly or peering through scrying sensors that resemble wide-open eyes surrounded by sparkling motes of light.
Halaster’s abilities far exceed those of most mortal wizards. His expertise with magic gates allows him to travel far and wide to engage in magical research. He spends much of his time creating gates, moving them around, and casting elder runes on them. Halaster’s gates connect the different levels of Undermountain, thus enabling him to bring new monsters into the dungeon to replenish those that die or escape. Even as groups of adventurers try to gain decisive control of just a small section of Undermountain’s halls, Halaster constantly alters the dungeon’s perils to thwart them.
Halaster’s true form is that of a tall, gaunt, male human, but he uses magic to take on many other visages and shapes. No matter what form he wears, the Mad Mage giggles and mutters incessantly. Contrary to appearances, however, Halaster is alert and attentive to the activities and preparations of all beings near him. He never willingly enters combat without first casting mage armor and mind blank on himself.

Biography

When he was young more than a thousand years ago, Halaster Blackcloak was known as Hilather, though where he spent his youth is never completely revealed. Some sources place him in the ancient Imaskari Empire. Whatever his nation of origin, it is known that he was more gregarious as a young man, creating and hosting magefairs and spellmoots, and taking many apprentices. However, as his power grew, he became more interested in aberrations and the mysteries of the Outer Planes, which led to various extradimensional travels that eventually left him unhinged with a growing paranoia, hunger for power and viciousness.
In 168 DR Hilather, now renamed Halaster Blackcloak, arrived at the foot of Mount Waterdeep with his team of apprentices, collectively called The Seven. Halaster summoned demons and other fiendish creatures from the outer planes to construct his headquarters and claimed the nearby lands as his own. Most likely he chose the location because of Sargauth Enclave, an abandoned Netherese outpost buried deep beneath the surface.
Halaster refused to release the monsters and demons after they constructed his new home, instead sending them into the caverns to explore. Thus began a long series of journeys into the area now called the Underhalls, driving out the drow, duergar, and various underdark monsters that lived there. By 309 DR Halaster had complete control over the near-endless tunnels and caverns and began constructing the largest and most deadly dungeon on the face of Faerûn, Undermountain.
By that time, Halaster had become completely mad and wanted nothing to do with the outside world anymore. He left his hold to live in Undermountain itself, allowing the keep to fall into ruin. Several of his apprentices came looking for him; he slew two before demanding the rest help him fortify his dungeon even more. One refused and managed to escape. Jhesiyra Kestellharp would later provide what little history there is about Halaster, the Mad Mage.