"Diabolus - Kikoku" is a dark fantasy anime series set in a world where humanity is plagued by demonic entities known as Diabolus. These creatures emerge from a parallel dimension called the Abyss, wreaking havoc and consuming human souls. The story centers on Kaito Arashi, a young man who becomes the host of a powerful Diabolus named Kikoku after a tragic encounter leaves him on the brink of death. Kikoku, unlike other Diabolus, retains a semblance of its former humanity and forms a symbiotic relationship with Kaito, granting him immense power but at the cost of his own soul being gradually consumed.

Kaito joins the Order of the Crimson Seal, an organization dedicated to combating the Diabolus threat. The Order is composed of individuals who have also bonded with Diabolus, using their abilities to protect humanity. Kaito’s unique bond with Kikoku draws attention from both allies and enemies, as his powers surpass those of ordinary hosts. The series explores the moral complexities of using demonic powers for human survival, as well as the internal struggle Kaito faces in maintaining his humanity while wielding Kikoku’s destructive capabilities.

The narrative unfolds through a series of missions undertaken by the Order, each revealing more about the origins of the Diabolus and the Abyss. Key characters include Aya Shirakawa, a skilled warrior and Kaito’s mentor, who harbors her own tragic past tied to the Diabolus; Renji Kurogane, a rival host whose Diabolus is driven by pure malice; and Sora Hoshino, a researcher within the Order who seeks to uncover the truth behind the Diabolus phenomenon. As Kaito delves deeper into the mysteries of the Abyss, he discovers a connection between Kikoku and a catastrophic event known as the Eclipse, which threatens to merge the human world with the Abyss.

The series balances action-driven plotlines with character-driven drama, exploring themes of sacrifice, identity, and the blurred line between good and evil. Kaito’s journey is marked by his evolving relationship with Kikoku, as well as his growing understanding of the true nature of the Diabolus and their role in the world. The story builds toward a climactic confrontation with the Abyss’s rulers, who seek to exploit Kaito’s unique bond to achieve their own apocalyptic goals.