Maou Gun Saikyou no Majutsushi wa Ningen datta is a Manga/Manhwa/Manhua in english language, Action series, english chapters have been translated and you can read them here. You are reading chapters on www.mgeko.nl fastest updating comic site. The Summary is

“Maou Army’s strongest magician” I’m called. Ike, a demon capable of capturing a fort alone. Clearing away enemies with immense magic, he is without a doubt a monster held in awe by friend and foe. However he has a secret he can’t tell other demons. “I, actually am a human you know.” Using modern knowledge to keep his secret, he became treated as a great commander and the driving force of the Maou’s army.