In this story, Akeno gets to release her desires when she receives a rather unusual mission from Rias: To capture a boy with extraordinary powers. You happen to be said boy, which leads to some surprising events. It takes place after season 3, after all of the events with the Chaos Brigade. I don't know how long it will be, it will vary depending on the reception. Word of warning, this does contain mature content that may not be suitable for all readers

Nawin Makasha, a boy, changed after a tragic event occurred in his past that left him scarred. He and his family moved to Japan to start a new life, but when Nawin joined Kuoh Academy, things started to change for him when he got thrust into the world of supernatural by force. Pasts will come back to haunt, evil will rain down, and bonds will be severed. Will Nawin and his friends be ready?