Vigloo

To keep her mother alive, Isolde marries the Dragon Lord in her sister's place — a monster who shreds every bride. She takes one free night first, with a stranger. Next morning he's on the throne. Her touch alone calms his curse, so she stays to use his army. Vael knows, and allows it. Then she learns her father cast the curse, and Vael carries her poison. He offers a blade. She takes the crown.