Halastar Blackcloak (formerly Hilather) was a very old and insane wizard who resided within his lair: Undermountain.

History 

In his youth more than a thousand years ago, Halaster Blackcloak was known as Hilather, though where he spent his youth is the source of great speculation. 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 Outer Planes, which led to growing paranoia and viciousness.
In 168 DR Hilather, now calling himself Halaster Blackcloak, arrived at the foot of Mount Waterdeep with his apprentices, collectively called “The Seven”. Halaster summoned demons and other outsiders to construct Halaster’s Hold 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 Hold, instead sending them into the caverns to explore. This began Halaster’s Hunts, a long series of journeys into the area now called the Underhalls, driving out the drow, duergar, and various 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 imaginable, Undermountain.
By that time, Halaster had become completely mad and wanted nothing to do with the outside world. 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 become Magister and would provide what little history there is about Halaster, the Mad Mage.

Elminster in Hell

In 1372 DR when Elminster was inadvertently sent to hell while defending the Dalelands from the Shadovar. Mystra made a bargain with Halaster, that she would try to cure his insanity if he rescued Elminster from hell. Halaster agreed, and entered Avernus. Though he was unsuccessful, and had to be rescued in turn by Alassra Silverhand, Mystra kept her word, and managed to partially cure him. (She then proceeded to blast Asmodeus with Halaster’s insanity.)

Halaster’s Apprentices

Several of Halaster’s apprentices would become extremely powerful in their own right, though all would become infected with his madness.
Shradin Mulophor  Shradin was a powerful Netherese necromancer who had survived the collapse of his civilization. In 1148 DR he learned of Sargauth Enclave and petitioned to become an apprentice to the Mad Mage. For reasons perhaps unknown even to him, Halaster agreed and also allowed him to take the Enclave as his own. Shradin, calling himself the “Lord of Bones” (not to be confused with the god of the dead who also used that title), turned the Enclave into Skullport, a neutral way-station in the Underdark, permanently linking Undermountain to the Lands Below. Trobriand  Called “the Metal Mage”, Trobriand has a gift for building metal constructs and monstrosities. Perhaps the most lucid and focused of Halaster’s apprentices, he nonetheless has moments where sanity fails him. Murial  Called “the Misshapen”, Murial was originally a great warrior who served as Halaster’s bodyguard. He later became an apprentice to the Mad Mage and used his growing power to augment his considerable strength and fighting prowess. Finally, as the madness of Undermountain overcame him, he changed himself into a centaur-like creature, but with a scorpion for a body. Though he is now a monstrosity, Murial remains Halaster’s most enthusiastic apprentice. Arcturia  Much like Murial, Arcturia’s madness leads her to experiment with bodies, both hers and others. Perhaps Halaster’s most ambitious apprentice, she is known for her cruel experiments with polymorphing others to suit her whims. Jhesiyra Kestellharp  For reasons unknown, Jhesiyra transformed herself into a living wish spell to attempt escape from Halaster, but was brought back to Undermountain by Halaster. She currently rules over other living spells in Undermountain’s Citadel of the Bloody Hand. Driven half mad by her transformation, she rules over the living spells the Mad Mage forced her to create, and it is uncertain whether she is truly aware of Halaster’s demise. Maeryln  An apprentice who was lost to time and history, all that is known of Maeryln is that she was a human who specialized in absorption of arcane powers, memories, and souls. It is rumored that Halaster took her on due to her resistance to the Spellplague that he somehow knew was coming. She was consumed by her own experiments when she summoned a lifedrinker specter that slowly drained away her memories and life-force. She existed only as a ghost for a long period of time and was tortured by said specter until recently “freed” by adventurers.

Halaster’s Higharvestide 

On Higharvestide of the Year of the Gauntlet, 1369 DR, two members of the Twisted Rune managed to kidnap Halaster Blackcloak from inside Undermountain. The lich Priamon “Frostrune” Rakesk and the alhoon Ralayan the Ocultacle used a ring of multiple wishes to summon the Mad Mage to Stardock where they could scrape the secrets of Undermountain from his mind. Halaster’s kidnapping allowed the madness of the dungeon to escape. Gates began randomly transporting creatures from Undermountain to places throughout the Realms, causing havoc and destruction everywhere. A group of adventurers hired by the Lords of Waterdeep managed to reach Stardock and free Halaster. The Mad Mage immediately returned to Undermountain, which quelled the chaos of the dungeon and ended the terror of Higharvestide.

Halaster’s Heirs 

While the term “Halaster’s Heirs” formerly referred to the surviving apprentices of Halaster, it currently refers to an arcanist academy on the Dungeon Level of Undermountain formed by Trobriand and Murial. 

Death 

Shortly after dawn on Eleint 30, 1375 DR, an earthquake struck Waterdeep. Though the city suffered little damage, many people throughout the town saw visions of Halaster screaming, his eyes ablaze with rage, sorrow, and swimming stars. Arcanists who saw the visions also reported scenes of destruction in the vast maze of Undermountain: pillars cracking and crumbling, chasms and rifts opening, and explosions of sparks. Those brave enough to investigate quickly discovered that Halaster had inadvertently killed himself while attempting a powerful ritual; as he died, he hurled desperate visions and compulsions to people all across the continent.

Personality

While in Undermountain, Halaster seems constantly distracted, always chuckling and muttering to himself over people and happenings only he can discern. He also has difficulty holding the thread of any conversation.
If he is ever removed from Undermountain, he regains his lucidity. At these times, Halaster is fastidious, meticulous, and quite proper, though still thoroughly evil. Halaster can be honorable, and even noble in a way. He brooks no insolence in any case, and remembers any slight or aid given him. He also tries to get his own way in everything, caring not for the harm he does to others in the process. Blackcloak’s interests include collecting new monsters and magic, but after thousands of years, nothing is really new to him. Whilst waiting for such delights, he amuses himself by manipulating events and politics to suit his whims.

Magic Items and Spells

Over many centuries, numerous magic items and spells have been created by or attributed to Halaster Blackcloak.
Magic Items 

• Halaster’s Drifting Portals
• Halaster’s Quaff
• Halaster’s Teleport Ring
• Horned Ring

Spells 

• Halaster’s Blacksphere
• Halaster’s Fetch
• Halaster’s Grappling Hand
• Halaster’s Image Swap
• Halaster’s Light Step
• Halaster’s Shaking Hand
• Halaster’s Teleport Cage

Timeline

-2488 DR  Lord Hilather, one of approximately three surviving Imaskari artificer lords, escapes the fall of Imaskar at the hand of the Mulhorandi and Untheric pantheons by entering a temporal stasis vault in a secret military stronghold beneath the present-day Giant’s Belt Mountains. 128 DR  Hilather (Halaster’s real name) awakens from temporal stasis after 2,620 years. Inhabits abandoned tower in Emirate of Torsil between Cloud Peaks and Candlekeep along the Sword Coast. 168 DR  Halaster and the Seven construct Halaster’s Hold. 171 DR  The beginning of Halaster’s Hunts. They last until 308 DR. 275 DR  Halaster discovers the Lost Level of Undermountain. 307 DR  The Seven abandon Halaster’s Hold for the Underhalls, leaving it to fall to ruin. 309 DR  Halaster establishes complete control over Undermountain. 326 DR  The Mad Mage finds the Crystal Labyrinth and begins reshaping it to his own ends. 1150 DR  Halaster grants Sargauth Enclave to Shradin Mulophor. 1325 DR  Events of Escape from Undermountain. 1369 DR  Members of the Twisted Rune kidnap Halaster, causing Halaster’s Higharvestide. 1372 DR  Events of Elminster in Hell. 1375 DR  Events of Expedition to Undermountain. Eleint 30  Halaster attempts a ritual of unknown purpose but great power. The ritual backfires, costing him his life and collapsing parts of Undermountain.