Guin Saga: The Seven Magi

From Vertical:

After awaking in a forest with a leopard mask attached to his head and no memory of his past, the peerless warrior Guin survives the treacherous wilds of Nospherus and the mean intrigues of the Middle Country.

Now, as King of Cheironia, he must contend with dark forces that dare to prey on his realm. King Guin must battle a plague made manifest and controlled by hostile magic. But even if our mighty hero vanquishes the Black Plague, how will he ever defeat the combined forces of powerful magi?

Based on the first Guin Saga Gaiden volume of the same name.