Demonbane (デモンベイン, Demonbein) is a 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 (機神咆吼デモンベイン) adaptation and a conversion to manga.
Kurou Daijuuji is a poor detective living in Arkham City. One day, he was requested by Ruri Hado of Hado Financial Group, to search for a magic book. While he initially refused, Ruri offered him a large sum of money upon completion of her request, in which bribed Kuro 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 Kuro with powerful magic. Soon afterwards, Al also activates Demonbane, a deus machina owned by the Hado Financial Group, to combat the mechanical menace from the Black Lodge. With this, the war between the Hado Financial Group and the Black Lodge begins….