The Hero who defeated the Demon King betrayed the Kingdom and became the new Demon King.On the final battle, he used an item for reincarnation, declaring that he will return.The Kingdom has already fallen.Tens of thousands of years have passed. The world has changed. The era of swords, magic, and demons has long been forgotten.It was a peaceful modern era.And now, the Fallen Hero who became the Demon King has returned.