The Lament of Man and Demon

The Human Emperor was defeated, and his soul was sealed by the Demon King and entered ten reincarnations. Since then, humans have become an inferior race of the Demons. Hundreds of years later, Allen, a human boy from a marginal village, was rescued by the demons. Within a few years, he passed the demons' war trials and was approved to drink the magic spring to transform from a human to a demon. The moment he drank the magic spring, he recalled his tenth reincarnation as the Human Emperor. On the one hand, he is his own savior, and he exchanges his blood for status and honor. On the one hand, it is his duty as the emperor, and he fights for the survival of the race. How would Allen choose? And later, what's the bigger conspiracy behind it?