Tainted Light

A hero faces the demon lord that tries to enslave the world along with his party. As the demon lord attempts to possess him, he desperately tries to resist. His party could not do anything but watch as he suffers in pain. As the hero almost loses himself to the demon lord, he damages himself so that he wouldn't be a threat to the party. In the end, the one that return is the hero himself, but he acts differently. What could have happened?