Kamigoroshi no Maou, Saijaku Shuzoku ni Tensei shi Shijyou Saikyou ni naru

Hiram, the strongest demon king in history and ruler of all demons, defies the Demon God's demands for sacrifices. This defiance leads to a fierce battle that leaves Hiram dead. Despite being the strongest, he could not overcome the truth of this world: Demons cannot defeat the Demon God. Five hundred years later, Hiram has finally reincarnated as a human. While humans are considered the weakest race, they actually have the highest potential for growth — and Hiram fully intends to take advantage of that fact.