Maou-gun Saikyou no Majutsushi wa Ningen datta (Light Novel)

I'm known as the Maou Army’s strongest magician. Like, I have the demonic power capable of capturing a fort alone. Clearing away enemies with immense magic, I am without a doubt a monster awed by friend and foe. However, I have a secret I can’t tell the other demons: I'm actually from Earth. Using modern knowledge to keep my secret, I'm being treated as a great commander and the driving force of the Maou’s army.