The Undefeatable Swordsman

Song Woo-Moon is the son of a simple inn owner in the countryside. His body is weak since childhood, and one day, a landscape painting received as a gift from an old Daoist causes his mind to become weak as well. For years, he is teased as being the village fool, and he grows up as nothing more than a pathetic burden to his parents. The day he turns twenty, however, his life changes! Something awakens within him—the supreme martial arts within the landscape painting! — Description from novelcodex.org