Devil Is a Part-Timer!, Vol. 21 (manga)

THE DEVIL KING IS HIRING! In the heart of the Bluehorns' fortress, a young Satan confronts the enemy chief, Adramelech. Defeating him would mean adding the clan's strength to Satan's army, but the Bluehorns are merely one step on the road to uniting the demons under a single banner. Years later, Satan will be challenged by an even greater foe-Alciel, leader of the Iron Scorpions! How did the future Devil King meet his most loyal general?