Maougun Saikyou no Majutsushi wa Ningen datta EP 12

In the army of Maou, there is a number one magician who can handle all the armies of demons all alone. He has great powers that he can clean all the enemies in just one spell. He has a dark secret tat he wants to keep as a secret from all others in the demon's world. He is a human whoo has extraordinary powers and he becomes a commander of the Maaou Army because of it.