The Shadowmaster was a wizard on Thundera who used a cloak of darkness to conjure his magical powers and attempted to take over the planet.
The Shadowmaster once attempted to destroy Thundera, but he was stopped and exiled by the young Jaga and Claudus with the help of the Sword of Omens. Many years later he returns from the exile offering to destroy the Thundercats to the Ancient Spirits of Evil in an attempt to gain their sympathy and displace the position of Mumm-Ra before them.
Through a sequence of nightmares, Lion-O eventually learns of his father's imprisonment in the Shadow Realm and is able to rescue him. The Shadowmaster has arguably come closer to defeating the ThunderCats than anyone else. In his one-time assault on the ThunderCats, the Shadowmaster would have succeeded had Mumm-Ra not interfered for fear that the Shadowmaster would replace him as the chief servant to the Ancient Spirits of Evil.
He had the ability to summon twisted creatures from shadow cape and can shoot mystic bolts.