She Chose the Dragon

Iso, the eldest princess of the Veridian Kingdom, had been hopelessly in love with Lyon, the captain of the Holy Knights, for ten years—until she caught him in a secret affair with her younger sister, Irene. Heartbroken, she volunteers to take her sister's place as the sacrifice to the dragon. Upon arriving at the dragon's domain, Iso discovers that the so-called brutal dragon, Kaelen—a thousand-year-old ancient dragon—is in fact lonely and kind.