The Mad Architect, originally a Breton man named Dutheil, was an infamous mortal architect during the mid-Second Era. He is said to be one of the few entities ever to serve multiple Daedric Princes and profit from it.
During his time on Tamriel, Dutheil worked as a brilliant but eccentric architect for the nobles of Wayrest. He cornered a rival architect, Gasteau Chamrond, out of countless professional contracts. The competition was so bad that Gasteau was unable to find work. Hoping to save his career, Gasteau worked with several of his business contacts to discredit Dutheil's reputation. This ended up destroying the man, driving him past the brink of madness.
Following the loss of his reputation, Dutheil took up the moniker of the Mad Architect and began to make pacts with the Daedric Princes. How the Princes paid for his "services" is a matter of wild and horrible conjecture. Following the Soulburst of 2E 578, he was contracted by Molag Bal to construct the Endless Stair in Coldharbour as the center of the Prince's attempted Planemeld with Nirn. He was also responsible for creating the Planar Vortex, where the two planes melded together. In the same time period, he remodeled the ruined Agea Relle with devices of eternal torture at the behest of Mephala for one of her favored mortals.
The Mad Architect subsequently took up residence in Coldharbour as a skeletal spirit, building the Vaults of Madness as his lair and masterpiece. The subterranean complex also served as a temple to Molag Bal in mockery of the Eight Divines. However, its primary purpose was as an eternal prison for his enemies, namely Gasteau and his accomplices. Their souls were imprisoned in the Vaults to be endlessly tortured. In 2E 582, a group of Undaunted adventurers became trapped in the Vaults. Gasteau's spirit offered to show them the way out if they helped fight through the Daedra and free the trapped souls. The group ultimately faced the Mad Architect in combat as a lich and defeated him.