Terrax is a character in Might and Magic VI: The Mandate of Heaven.
Terrax was Enroth's first archmage. By increasing his powers with a powerful crystal, he laid the foundations of what would later become elemental magic, as well as light and dark magic. From then on, every new archmage on the continent would be guided by the study of the Crystal of Terrax.
Terrax moved to Castle Darkmoor, where he continued his studies as a lich. He never left the castle again, and at the time of the Night of Shooting Stars, many believed that he was still in there, researching new spells.
It's possible that the lich king in Castle Darkmoor is meant to be Terrax, but this is never stated outright.