Kishin Houkou Demonbane

This was the world where magical theory had advanced as well as science. Arkham City prospering by magic technology was threatened by a criminal syndicate Black Lodge. The Black Lodge was led by Master Telion, who was the most powerful wizard in the history, at the same time, the cruelest and the worst wizard in the history. The devotees of Black Lodge were giving free rein to their desires, and they were devastating and confusing the city.

Their robots destroyed the city completely, and the wizards were wielding their evil magic. The worst of all, they had Deus Machina, which was the strongest magic robot summoned by the evilest magic. No one could match Black Lodge.
Hado Family group, which supported Arkham city, made a robot to battle against Black Lodge. The robot, Demonbane, was the strongest robot made by the magic technologies.

However, a magic book containing strong power was necessary to start Demonbane.

Daimonji Kuro who was a poor detective was asked to look for a magic book by Hado ruri who was the leader of Hado family. Because he had a trauma, he was reluctant to be involved in magic affairs at first. But, the reward she offered was so much that he could forget his trauma.

Then, Kuro met a girl named Al, who was chased by Black Lodge. To be surprised, she was a sprit of the magic book Al Azif. They ran away from the Black Lodge, and they met Demonbane sleeping under the city where they escape to.

pls credit animenfo.com for summary

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[One episode - OVA (Shinsen fansubs); see also: Demonbane TV]