Kimi to Boku no Saigo no Senjou, Aruiha Sekai ga Hajimaru Seisen 1x1

Two nations, the Empire and the Nebulis Sovereignty, have been at war for 100 years. Imprisoned for freeing a witch, Iska is given a chance at freedom. In exchange, he must take down a powerful astral mage known as the "Ice Calamity Witch."