The Seventh Bride — T. Kingfisher

Young Rhea is a miller's daughter of low birth, so she is understandably surprised when a mysterious nobleman, Lord Crevan, shows up on her doorstep and proposes marriage. Since commoners don't turn down lords -- no matter how sinister they may seem -- Rhea is forced to agree to the engagement.

Lord Crevan demands that Rhea visit his remote manor before their wedding. Upon arrival, she discovers that not only was her betrothed married six times before, but his previous wives are all imprisoned in his enchanted castle. Determined not to share their same fate, Rhea asserts her desire for freedom. In answer, Lord Crevan gives Rhea a series of magical tasks to complete, with the threat “Come back before dawn, or else I'll marry you.”

With time running out and each task more dangerous and bizarre than the last, Rhea must use her resourcefulness, compassion, and bravery to rally the other wives and defeat the sorcerer before he binds her to him forever.