Ossan Boukensha no Okureta Eiyuu-tan: Kansha no Suburi wo 1-nichi 1-man Kaishite itara, Kensei ga Deshi-iri Shigan ni Yatte Kita

Guy Gullveig was once bullied by his half-brother for being an illegitimate child. Eventually, he was taken in by his grandfather, a former adventurer living deep in the mountains. He prescribed "swinging the sword" to Guy as a way to heal both his body and mind. Even after his grandfather's death, Guy continued to practice 10,000 swings every single day, eventually reaching middle age. One day, Guy decides to go to the city to become an adventurer, following his late grandfather's advice. But what Guy did not know is that those years of practice swings have turned him into the strongest swordsman alive!! From the Sword Saint Altina to the guild receptionist Lilina and the feudal Lady Celis, the people Guy helps out of the kindness of his heart begin to realize how strong he actually is. This is an "unconscious unrivaled" fantasy story about an old adventurer who worked a little too hard on his daily 10,000 swings and now strikes down the strongest with his perfected blade!! (Source: Square Enix, translated)