The Undefeatable Swordsman

The son of a lowly innkeeper, U-Mun Song always dreamed of being a martial arts master, a dream that would never become a reality thanks to his frail body. Then one night, a mysterious guest leaves behind a painting as payment that enthralls U-Mun for reasons he can’t explain. It soon becomes clear that it’s no ordinary painting when U-Mun starts showing prowess in the martial arts he’s never had before. Thus begins the path for a scatterbrained kid to change his destiny and become truly undefeatable!

(Source: Tapas, edited)