William was one of the most powerful person in the world that could even matched up a god. One day when he was slaughtering his way to the demon's army the seven demon King's and the four divine beast ambushed him that lead him to fight this powerful foes on his own. In the end William managed to kill three demon King's before he died in the battle.
When he opened his eye's he found out the he travel back in the past before the war against the demon's.
