Eberron - Dreams of Xen'drik
Centuries ago, a powerful necromancer named Keldor attempted an uprising against the matron of the Han’tar’kul Drow clan. His efforts failed, and he and his supporters retreated to the relative safety of the tunnels beneath Bazek Mohl. Not one to forgive a slight, the Matron sent assassins and warriors in after him, until one group finally returned and confirmed that that the wizard was no more. However, years later patrols and scouting parties into the tunnels began to disappear again, and word slowly leaked back that Keldor was still alive in the tunnels – or if not alive, he was still active. Before the Matron could mount a serious attempt to discover the truth, she was routed by the return of Melraevel Pilar’kah, who deposed her and ended her life, leaving Keldor to build his power beneath the city until a group of adventurers encountered him, destroying many of the undead he had created and disintegrating his physical form.
Keldor was the name of an ancient foe of King Randor of Eternia. Keldor was defeated by Randor, and before his death he begged the aid of a powerful outsider named Hordak, who transformed him into the villain known as Skeletor.