Sora ni Hibiku wa Ryuu no Utagoe

Called by a mysterious voice, unconscious Ryuusei Moriya wakes up in an unfamiliar place. It was Elman, a country in another world where dragons soar in the sky and a dragon king ruled. “According to an ancient contract, you have to live as consort to the dragon king….”, was what he was told. And then, a crimson-haired dragon king, Fei Wan, appeared before a confused Ryuusei... Source: NU