"Demonbane" is a 12-episode anime series based on the visual novel by Nitroplus. The story is set in the fictional city of Arkham, a place steeped in occult mysteries and supernatural phenomena. The narrative centers around Kurou Daijuuji, a young and struggling private investigator with a penchant for the occult. Kurou's life takes a dramatic turn when he encounters Ruri Hadou, a powerful sorceress and the heir to the Hadou financial empire. Ruri is pursued by the sinister Black Lodge, a secretive organization seeking to harness the power of ancient grimoires for world domination.

Kurou and Ruri form an unlikely alliance after discovering the existence of Demonbane, a sentient, ancient mecha powered by the Necronomicon, a legendary grimoire. Demonbane is a weapon of immense power, capable of combating the eldritch horrors unleashed by the Black Lodge. As the series progresses, Kurou and Ruri must navigate a complex web of alliances, betrayals, and ancient prophecies. They face off against a variety of adversaries, including other sorcerers, rogue mecha, and eldritch abominations, all while uncovering the deeper mysteries surrounding the Necronomicon and its connection to the fate of the world.

The plot is driven by the escalating conflict between the protagonists and the Black Lodge, led by the enigmatic Master Therion. The series explores themes of power, destiny, and the consequences of wielding forbidden knowledge. Kurou's growth as a character is central to the narrative, as he evolves from a reluctant participant to a determined protector of humanity. Ruri, on the other hand, grapples with her own legacy and the weight of her responsibilities as a sorceress.

The anime blends elements of mecha action, Lovecraftian horror, and occult fantasy, creating a unique narrative that balances intense battles with moments of introspection. The relationship between Kurou and Ruri is a key focus, with their partnership serving as the emotional core of the story. The series concludes with a climactic showdown that ties together the various threads of the plot, leaving a lasting impact on the characters and the world they inhabit.