The Hero, in the Past Tense

In 2077, the Demon King from another world descended upon the human realm with his Four Great Generals, bringing the concept of magic to a world that had once been entirely mundane. Roselia, one of the Four Generals, created tens of millions of demons, recreating the conflict between demons and humans. Forty-three years of defensive warfare, with battles on land, at sea, and in the air: there’s no word for it but tragic. The flames of war spread across all seven continents and four oceans, devouring humanity’s living space and transforming the way people lived.