Sensen Senki

In a turbulent era where numerous nations divided the world, a small country was conquered and destroyed by an ambitious warlord, and a young boy was taken prisoner. But the boy did not give up... he vowed to one day save his people from the warlord's clutches. To that end, the boy aspired to become a being with supernatural powers, a "sage"!

(Source: Kodansha, translated)