Maken Kajishi ni Naritakute!

Eiji, a hero from Holy Sword Academy, was recognized as the strongest hero and qualified to hold the "Supreme Holy Sword," a holy sword among the holy swords. However, dissatisfied with the arrogance of the Holy Sword Academy, Eiji ran away. He quits his position as a hero and becomes a commoner, trying to create a sword that matches his abilities with his own hands. Finally, after much trial and error, he arrives at the answer, to create the most powerful sword using the bodies of the monsters as the materials.

(Source: Shousetsuka ni Narou, translated)