The Strongest Magician in the Demon Lord's Army was a Human

Maou Gun Saikyou no Majutsushi wa Ningen datta• 魔王軍最強の魔術師は人間だった

Ike is a powerful magician and the leader of the Immortal Brigade, part of the Seventh Corps of the Demon Lord’s Army. He single-handedly conquers fortresses and pushes back the armies of humanity. Neither Dairokuten, the Demon Lord, nor Ike’s loyal soldiers know his darkest secret—he is a human in hiding! But can he keep his secret safe and bring peaceful coexistence to demons and humans?

(Source: Crunchyroll)