Mongul

Mongul was the tyrannical ruler of Warworld, an artificial satellite that traveled the galaxy picking up slaves for gladiatorial combat. Superman was picked up by a Warworld slave ship, where the Man of Steel began winning match after match. Mongul finally pitted Superman against the warrior Draaga, one of Warworld's finest. But when Superman refused to kill his foe, Mongul decided to fight Superman himself. Mongul used his gemstone's power to overwhelm Superman, but fortunately the Cleric teleported Superman away from the fight. As Warworld's inhabitants began to revolt against their hated ruler, Mongul was defeated by Draaga and replaced as ruler of the planet after Draaga learned how to overcome his gemstone. Mongul fled, swearing revenge.