"Urotsukidoji: Legend of the Overfiend" is a dark fantasy anime series that intertwines elements of horror, erotica, and supernatural action. The narrative is set in a world where three realms—the human world, the demon world, and the beast world—exist in a fragile balance. The story centers on the prophecy of the Overfiend, a powerful being destined to unite these realms and bring about a new era of chaos or salvation.

The protagonist, Amano Jyaku, is a seemingly ordinary high school student who harbors a hidden connection to the demon world. Unbeknownst to him, he is the reincarnation of the Overfiend, a figure of immense power feared by both demons and humans alike. As Jyaku's latent abilities begin to awaken, he becomes the target of various factions vying to control or destroy him. Among these factions are the demon lords, who seek to manipulate Jyaku to maintain their dominance, and the human resistance, which views him as a threat to their survival.

The plot unfolds as Jyaku is drawn into a series of violent and surreal encounters with demons, beasts, and other supernatural entities. His journey is marked by intense battles, grotesque transformations, and moral ambiguity. Along the way, he encounters allies and adversaries, including Nagumo, a demon hunter with a personal vendetta, and Megumi, a human girl whose fate becomes intertwined with Jyaku's. The narrative explores themes of power, corruption, and the blurred lines between good and evil as Jyaku struggles to understand his role in the prophecy.

As the story progresses, the boundaries between the three realms begin to collapse, leading to apocalyptic events that threaten the existence of all worlds. Jyaku's transformation into the Overfiend becomes inevitable, culminating in a climactic confrontation that determines the fate of the realms. The series delves into the consequences of unchecked power and the cyclical nature of destruction and rebirth, leaving a lasting impact on its characters and the world they inhabit.