Just like the red dragon who descended from heaven, the royal bloodline of oni have arised from the underworld to the surface of the human world. Fearing that his bloodline would cause catastrophe among the humans, the Underworld King sealed their powers under a seal as a contract to be able to live in the human realm. 2000 years ago, it was said that an oni stood beside the red dragon, the first king of Kouka. 2000 years later, it's reincarnation was born into the descending bloodline of the ancestral oni, this young oni is Izanami (Y/n).