My Husband is Weak by Day, Strong by Night

Meredith, the scorned illegitimate daughter of a duke, is forced into a political marriage with the Emperor of Valtzein. She soon discovers he suffers from a mysterious curse, leaving him frail by day but powerful and dangerous at night. After he attempts to kill her, she proposes a bargain: she will use her healing abilities to break the curse in exchange for her freedom. But as she works to cure him, the Emperor finds himself captivated by her strength and resolve, and he has no intention of letting her walk away once the curse is lifted.