Orthos

Orthos was the true king of Filtwood Kingdom. He was a nameless demon who was thought to be sealed away by a champion named Orthos.

Background
Orthos was one of the few cases of self-naming. As a man-made hero, the voices of his nation's people gave him power, that power gave him a name allowing him to become a named being on his own.

He, later on, tried to trick and kill adventurers in order to gain more power but was destroyed by another demon never to regenerate again.

It is revealed by Kuro (aka Diablo) that he used to be an underling of Rouge (aka Guy Crimson).