The Wandering Overlord is one of the Yama Kings.
Potentially the most dangerous and mysterious of the Yama Kings, the Wandering Overlord is the one who has no domain, but is said to live just as well in both Yomi and the Middle Kingdom. He is unaffected by the conditions found in any of the Thousand Hells, and has never been directly bested by his brethren. Neither does he gather souls, the source of his power unknown to even the most ancient of spirits.
Some say the Wandering Overlord is Yama: Death itself. Others still believe that he is the true Demon Emperor. His shape is ever changing, often taking the guise best suited when dealing with a particular individual. Because of this, few can tell if the figure is even truly male, perhaps having no gender to begin with. All that is certain is that where the Wandering Overlord goes, chaos and corruption soon follow.