"Chōjin Densetsu Urotsukidōji: Kanketsu-hen" serves as the concluding chapter of the "Urotsukidōji" series, a dark fantasy anime rooted in supernatural and apocalyptic themes. The narrative centers on the struggle between humans, demons, and otherworldly entities, set against the backdrop of an impending cataclysm that threatens to merge the human world with the demon realm. The protagonist, Tatsuo Nagumo, is a young man who becomes entangled in this cosmic conflict after encountering Amano Jyaku, a powerful demon seeking to open the gates between dimensions. Tatsuo is revealed to be the reincarnation of Urotsukidōji, the Overfiend, a being destined to bring about the destruction and rebirth of the world.

The story unfolds as various factions vie for control over the Overfiend's power. The human world is plagued by chaos as demons infiltrate society, exploiting human weaknesses and spreading terror. Meanwhile, the demon realm is equally fractured, with factions led by Amano Jyaku and the demon lord Akuma seeking to harness the Overfiend's abilities for their own ends. Tatsuo's journey is marked by his internal struggle to reconcile his humanity with his demonic destiny, as well as his relationships with other key characters, including his love interest, Megumi, and the enigmatic demoness Suikakuju.

As the narrative progresses, the boundaries between the human and demon worlds begin to collapse, leading to widespread devastation. The climax revolves around the final confrontation between Tatsuo, now fully transformed into the Overfiend, and the forces seeking to control or destroy him. The series explores themes of power, corruption, and the cyclical nature of destruction and creation, culminating in a resolution that underscores the inevitability of change and the cost of transcendence. The characters' fates are intertwined with the fate of the worlds they inhabit, resulting in a conclusion that is both tragic and transformative.