Demonbane

Demonbane (デモンベイン Demonbein) is a Nitroplus series 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 has been released by JAST USA while the anime is streamed with English subtitles by Crunchyroll.

Plot
Kuro 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. While being chased by the Black Lodge, Al forges a contract with Kuro, bestowing him 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....

Other Sources

 * (Anime)