Devil Is Part Timer Graphic Novel Volume 21

In the heart of the Bluehorn clan's fortress, a young Satan confronts their chief, Adramelech. Defeating him would mean adding the Bluehorn's strength to his army, but can he surmount the overwhelming difference in power?! Years later, a now adult Satan will come face to face with Alciel, leader of the Iron Scorpions. How did the Demon King meet his most loyal general?