Maou Tsukai No Saikyou Shihai summary:

For incompetence, Lewin, a Demon-Tamer adventurer, was banished from the hero’s party. Without being acknowledged for his achievements as a commanding officer...However, the moment he comes into contact with Sasha, the Demon King of Death, one of the sealed Seven Demon Kings, he awakens to become a Demon King with the ability to subjugate Demon Kings! Moreover, by shattering the seal, he tames the Demon King, Sasha——Lewin, backed by Sasha, succeeds in drawing out the Demon King’s power and works to subdue all Demon Kings, the root of the strife between the humans and the demons.An overwhelming fantasy of warriors by the boy who rules the Demon King begins.