The Hero Life of a (Self-Proclaimed) Mediocre Demon! 4 by Shiroichi Amaui

He may look like nothing but a country boy, but Chrono Alcon is actually one of the most powerful demons the world has ever seen! All he's interested in is attending school like any other kid, but when the Demon King finds out about Chrono's power, he'll have to spend his days studying, and his nights in the dungeons, fighting monsters...?!
Binding: Paperback / softback