Pre-canon. How Fuuga came to part, and how Kazuki came to leave.
Saizou named him Prince of Terror. Kazuki never objected. It was true enough, and if some of the terror that had lodged in his bones for years seeped out to touch the people they fought, the ones who threatened even the tiny corner of life he had managed to cling to here… well, perhaps that would mean less for him which was all to the good as far as he was concerned.