Maou Tsukai no Saikyou Shihai

Lewin, a Demon-Tamer adventurer, is expelled from the hero’s party for being considered incompetent, despite his achievements as a commander. His fortunes change when he meets Sasha, the Demon King of Death, one of the sealed Seven Demon Kings. Their encounter awakens Lewin’s ability to become a Demon King and subjugate other Demon Kings. After breaking Sasha’s seal and taming her, Lewin draws on her power as he sets out to subdue the Demon Kings, whose conflict lies at the root of the strife between humans and demons.