Set after High School DxD Hero, things seem to being going well at Kuō Academy, on the surface. But of course, the normal students remain unaware of the supernatural events surrounding the school. When a new supernatural anomaly who claims to be Kiba Yūto's brother poses a threat to devils, angels and fallen angels alike, both the Occult Research Club and the Student Council seen dead set on eliminating it. But things take a dark turn when Kiba threatens both parties to leave the threat to him personally. What connection could Kiba have to this new foe? Will he turn against his comrades for the sake of it? Will Tsubaki Shinra remain dutiful enough to oppose him should the time arise?