The Underking is one of the five monarchs and the ruler of all undead creatures. He resides in the Under Realm, and anyone that looks upon the Underking instantly becomes undead. When the Underking needs to speak with someone away from his chamber, he uses Bardox the revenant as an intermediary. Gazing upon Bardox does not turn the watcher into one of the undead.
In addition to transforming his visitors into the undead, The Underking is sufficiently powerful to eavesdrop on conversations far from the Phantom Isle. Uninvited demons and dragons would fall in his presence.
Seth, with his memory lost, was taken by Ronodin to meet with the Underking as a student of the dark unicorn. The Underking tasked him to liberate the Blackwell. With the aid of the Sphinx, Seth freed the undead prisoners, causing Wyrmroost to fall.
Kendra and the delegation of Knights that go to help restore Crescent Lagoon learn that the Sunset Pearl, the primary source of the sanctuary's protection, had been taken. It was later revealed that the Underking was in possession of it until Kendra recovered it, finally seeing the Underking's true form as a decrepit skull and two discarded bones. Atop the skull is a crown that reflects absolutely no light.
In the duel between Ronodin and Bracken, Ronodin revealed that the Underking had two of his three corrupted horns, which he obtained by arranging for Seth to open the Blackwell, and retrieving the Sunset Pearl.