Tatsuo Nagumo is a human high school student destined to father the Chōjin (Overfiend), the supreme being prophesied to unite the human, demon, and man-beast worlds. Initially shy and lecherous, his ordinary existence shatters when supernatural forces recognize him as pivotal to the apocalyptic convergence of the three realms. His background establishes him as an unassuming teenager until latent power erupts, transforming him into the "Demon of Destruction." This mutation unleashes catastrophic devastation, clearing the path for his messianic offspring's birth. In *Legend of the Demon Womb*, his cousin Takeaki Kiryū becomes possessed by the ancient entity Kyō-Ō (Lord of Chaos), orchestrated by the antagonist Münchhausen II. This event forces Tatsuo into direct conflict with his corrupted relative. Tatsuo's relationship remains centered on Akemi Itō, his girlfriend and the future mother of the Chōjin. Akemi’s inherent purity and connection to the Overfiend grant her near-invulnerability, shielding her from harm except by the Kyō-Ō’s power. His interactions include Megumi Amano, a man-beast who initially mistakes him for the Chōjin and later develops a protective dynamic with his family. His powers manifest involuntarily during extreme arousal or danger, triggering violent transformations into a demonic entity. These episodes often inflict death or sexual violence on those nearby, with Tatsuo retaining no memory afterward. As his transformation progresses in the sequel, he loses control over his human identity, becoming an instrument of apocalyptic change.