Why is the Demon King like this?

Conquering another world, beating up heroes, and subjugating all nations. The restorer of the demon race, the greatest mage in the world, the gravedigger of the goddess girls. He vowed to break the scepter of the supreme goddess and plant the demon banners throughout the divine realm. From then on, all races would submit, achieving the great feat of unification! Ah… none of this will happen. Because the Demon King has already been slain by the hero (sadly). Therefore, this is a story of a demon king transmigrating. Keywords: Pseudo-Tokyo, Yokai