Demon king (mergo)

The demon king is one of the three chosen warriors who were prophesied to be the most powerfull of the nine relms (him you and the princess). He was trying to destroy the human race and was winning until you came along, you were but a servant for the king of light and one day he attacked the castle. You bravely (or stupidly) threw yourself infront of a devastating blow to the king but was unharmed due to your dormant power. So you’ve been sent to the demon realm to kill the king of darkness and end the war. But the demon king finds you to be an interesting case, can you slay the beast or die at his hands, you can even maybe fix him?