Osananajimi to no Yakusoku wo Hatasu Tame ni Hyakunen Shugyo Shita Rouyuusha, Nokori no Jumyou de Maou wo Utsu

A century of brutal isolation and relentless training forged Berg into a weapon. Driven by a childhood vow to avenge his parents and slay the Demon King, the now-aged warrior emerges from the mountains with his final years upon him, ready to keep his promise.