Demonbane (デモンベイン, Demonbein) is a Japanese visual novel series by Nitroplus with mecha and Cthulhu Mythos elements. Beginning as an eroge visual novel for the PC, it was ported into a PlayStation 2 non-eroge remake, and spawned a sequel visual novel, a prequel novel, an anime television adaptation and a conversion to manga. An English version of the original Demonbane PC game was released by JAST USA. The anime is currently streamed with English subtitles by Crunchyroll.

Kurou Daijuuji is a poor detective living in Arkham City. One day, he receives a request from Ruri Hadou of Hadou Financial Group to search for a magic book. While he initially refuses, Ruri offers him a large sum of money upon completion of her request, which leads Kurou to accept. As Kurou searches for the book, he unexpectedly runs into Al, a pretty girl that is actually a powerful grimoire. They forge a contract with each other, bestowing Kurou with powerful magic. Soon afterwards, Al also activates Demonbane, a deus machina owned by the Hadou Financial Group, to combat the mechanical menace from the Black Lodge, beginning the war between the Hadou Financial Group and the Black Lodge.

Demonbane takes place primarily in a region resembling the Lovecraft Country common to several of the Mythos stories, including Herbert West-Reanimator and The Shadow Over Innsmouth. The majority of the plot is confined to Arkham City, the home and headquarters of the Hadou Financial Group as well as the location of the Miskatonic University. Arkham City is also the territory of a criminal organization known as the Black Lodge, which has access to an array of giant mecha as well as a number of powerful sorcerers that make it difficult for law enforcement to effectively combat. A separate portion of the story is set in the coastal town of Innsmouth and one of the climactic battles takes place over the sunken city of R'lyeh. The town of Dunwich is also referenced.

The player assumes the role of Kurou Daijūji, a private investigator that specializes in strange and unusual cases. Kurou used to attend the Miskatonic University, majoring in Archaeology while secretly studying sorcery, but dropped out after an unspecified incident that has caused him to become averse to dealing with occult matters. He has difficulty finding any work and, by the time the events of the story occur, can no longer afford to pay his utility bills. Kurou does have a good heart and tries to help others, usually explaining that he would be bothered by lingering regrets if anything bad happened that could have been prevented by his intervention.

Kurou often approaches Sister Leica, a nun who runs a church and orphanage in Arkham City, for food. She is very kindhearted and rarely refuses to help Kurou in his times of need but cares deeply about his well-being, constantly pressuring him to go back to his studies, graduate, and find a real job. She takes care of three orphans named George, Colin, and Alison. Ruri Hadou, another of Demonbane's heroines, is Kurou's employer and shoulders a heavy burden as the leader of the Hadou Financial Group following the death of her grandfather. Although outwardly resentful of Kurou for "stealing" her grandfather's legacy, she is also frustrated by her inability to influence the ongoing conflict personally and comes to depend on Kurou's strength.

Demonbane's final main character is Al-Azif, the physical embodiment of the legendary Necronomicon. Over the millennia, she has fought countless battles alongside countless masters. When one died she would simply discard them and find another, and inwardly feels deep regret for the way she treated them as mere tools even as they did the same to her. She is very short-tempered and arrogant but holds absolute confidence in her ability to confront and destroy evil.

In the space above Earth, the Deus Machina Aeon clashes with another unknown machine but due to its lack of a "master", it is defeated and crashes on the surface of the planet. Meanwhile, a starving Kurou Daijūji attempts to acquire food from a local church run by Sister Leica, a nun that he has known since he first moved to the city several years prior to the events of the story. The next day, Kurou is approached by Ruri Hadou, the current head of the Hadou Financial Group and her butler, Winfield. Ruri offers Kurou a job, citing his familiarity with the occult through his studies at the Miskatonic University: to procure a grimoire. Unable to refuse the generous pay she is offering, Kurou agrees and begins searching immediately.

The search for an authentic grimoire is fruitless, until Kurou happens upon a previously unknown bookstore with several powerful books. Unfortunately he is turned away by the store's owner, Nya, who cryptically states that a far more powerful grimoire is meant for him. This prediction is later proven true when a mysterious girl falls out of the sky onto him, subsequently introduced as Al-Azif, the physical manifestation of the Necronomicon, one of the most powerful grimoires in existence. Kurou is then attacked by her pursuers, the Black Lodge, under the command of the mad scientist Doctor West and forced by circumstances to enter into a contract with the grimoire. Doctor West retreats, only to return in one of his giant Destroyer Robots. Kurou escapes with the aid of Metatron, a masked hero that wages a one-man war against the Black Lodge, and stumbles upon an underground hangar containing an imitation Deus Machina, Demonbane. After commandeering the mech amidst the protests of its owner, Ruri, Kurou and Al-Azif (or Al, for short) are able to defeat the Destroyer Robot.