"Oninagi" is a supernatural manga series that blends elements of action, horror, and dark fantasy. The story centers around Nanami, a high school girl who becomes the vessel for a powerful and ancient demon named Onigami. Onigami, once sealed away by a clan of demon slayers, is unleashed when Nanami accidentally breaks the seal. As the host of Onigami, Nanami gains immense power but also becomes a target for both humans and demons who seek to control or destroy her.

The narrative explores the conflict between humans and demons, as well as the moral complexities of wielding such destructive power. Nanami struggles to maintain her humanity while navigating her new role as a demon vessel. She is aided by a group of allies, including her childhood friend Tsubasa, who is a member of the demon-slaying clan responsible for sealing Onigami. Tsubasa is torn between his duty to eliminate demons and his desire to protect Nanami, creating tension within their relationship.

Throughout the series, Nanami encounters various demons and supernatural entities, each with their own agendas. Some seek to exploit her power, while others aim to destroy her to prevent Onigami from wreaking havoc. The story delves into themes of identity, loyalty, and the consequences of wielding power beyond human comprehension. As Nanami battles both external threats and internal struggles, she must confront the true nature of Onigami and the dark history that binds them together.

The manga features a mix of intense action sequences, emotional character interactions, and a richly developed supernatural world. The plot is driven by Nanami's journey to understand her connection to Onigami and her efforts to protect those she cares about while grappling with the destructive potential of her newfound abilities. The series balances its darker themes with moments of camaraderie and personal growth, creating a compelling narrative that explores the boundaries between humanity and monstrosity.