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

Driven by the demons who murdered his parents, Berg begins a journey to defeat the Demon King. On the night his village is attacked, he makes a solemn promise to his childhood friend: he will slay the Demon King. To fulfill that vow, Berg retreats into the mountains and dedicates himself to relentless training. A full century passes. Now 110 years old, Berg finally leaves his mountain seclusion, armed with punches of overwhelming power. His goal remains unchanged as he sets out toward the Demon King. Along the way, he encounters Eruruna, an elven swordswoman engaged in battle against the demons. The story follows Berg's resolve to honor a promise made long ago, his extraordinary strength forged through a hundred years of isolation, and the path that brings him into conflict with the Demon King's forces, beginning with his meeting with Eruruna.