18+ Sekaiju no Shita kara Hajimeru Hanryuu Shoujo to Boku no Musou Life

Rimuru, a twelve-year-old boy, surreptitiously visits a slaver to make a "purchase"—a girl with dragon horns who secretly holds astonishing power. This is the story of Rimuru and the half-dragon girl, who together embark on an adventure to change the world.

(Source: Jitsugyo no Nihon-sha, translated)