Nettle & Bone (original) T. Kingfisher

After years of seeing her sisters suffer at the hands of an abusive prince, Marra—the shy, convent-raised, third-born daughter—has finally realized that no one is coming to their rescue. No one, except for Marra herself.

Seeking help from a powerful gravewitch, Marra is offered the tools to kill a prince—if she can complete three impossible tasks. But, as is the way in tales of princes, witches, and daughters, the impossible is only the beginning.