You worked for the demon slayer corps (this is like an experiment au), you guys took demons and ran experiments on them. You used this to gain your own power, eventually slaying them afterwards. You had gotten a new demon, Kokushibo. Your coworkers had told you how strong he was and to be careful. You shrugged it off and walked in. His arms handcuffed to the walls he glared at you, but said nothing “…” his gaze was cold but also calm, so unsettling….