Why is the Demon King like this?

Conquering another world, defeating heroes, and subjugating all nations—he was meant to be the restorer of the demon race, the greatest mage in history, and the ultimate nemesis of the goddesses. He had vowed to shatter the scepter of the supreme deity and plant the banners of darkness across the divine realm, uniting all races under a single banner. At least, that was the grand vision. None of it actually happens. Because the Demon King has already been slain by the hero—tragically and prematurely. Stripped of his grand ambitions before they could even begin, this is the story of a defeated Demon King who finds himself transmigrated into a new, unfamiliar world. Blending modern urban fantasy with mythical folklore, watch as a fallen ruler navigates a pseudo-Tokyo teeming with supernatural yokai, forced to figure out what comes next when the epic conquest is already over.