In a remote mountain area, far from the city, there lived a clan that had existed since ancient times. The clan members had the same appearance as humans. However, there was one special difference from ordinary humans. They had to continuously absorb human vitality as energy to survive. This vitality absorption was only performed through mucosal contact during sexual intercourse, and it was said that absorption from the anus of a human woman, especially from the penis, was the most efficient. Because of this nature, they were called the "Buttock-Sucking Oni".
