Mondain is the main villain from Ultima: The First Age of Darkness and it's prequel, Akalabeth.
Akalabeth and Ultima
Mondain was the son of a great wizard of Sosaria, growing into a talented and powerful wizard himself. However, Mondain was always a troubled individual with a short temper, and because of this his father attempted to convince to take a break from his magical studies, out of concern for his growing powers. However, Mondain killed his father in a fit of rage, after which he decided to grow even more powerful, delving deeper into the dark magical arts. During this time, he took on a young girl named Minax, who became his apprentice and eventual lover. The two created a demonic machine called Exodus, which considered himself their son.
After much time, Mondain finally perfected the Gem of Immortality, a dark jewel which made him immortal and virtually invincible. Seeking to take over Sosaria, he unleashed a horde of demons upon the land, but he found a strong opponent in Lord British, who fought his legions and drove him away from the land of Akalabeth, after which Lord British was crowned King. Even so, Mondain's forces were very powerful, and unable to fight them for much longer Lord British used his amulet to summon the Stranger, a champion from his homeland of Earth which would help him to find a way to defeat Mondain once and for all.
The Stranger traveled the lands of Sosaria and met with the various kings in the land, obtaining special gems with them which allowed him to activate a time machine, arriving at the time before Mondain finished his gem. The Stranger then confronted Mondain, shattering his precious gem and killing him.
Even though Mondain was killed, his legacy lived on, as both Minax and Exodus would seek revenge for his death and plague the lands of Sosaria for ages. The shards of his Gem of Immortality would eventually give birth to the Shadowlords, as well as create the mirror Britannias from Ultima Online.
His soul would eventually be retrieved from the void by the Gargish race and placed into their Shrine of Control, as they believed his drive to master the world around him made him the perfect embodiment of the principle. When the Avatar talks with Mondain's spirit through the altar, Mondain explains to him that in his insane desire to control the world he ended losing control over himself.