Oh, yes, you read that right.
That's the character at the heart of Grave Mercy, by Robin LaFevers. Ismae had a fairly bad early life. Born in 15th century Brittany (a tiny European country, now part of France) to an abusive father and a mother who tried to abort her, she was married very young to an equally abusive husband. When she ran away, she discovered the convent of Saint Mortain, where nuns live in service to their patron saint and, by the way, also learn the arts of murder. That's because Saint Mortain is the new name for the old god of death, and he helpfully marks the people that he wants his nuns to kill.
After a few years and a lot of skills, Ismae is sent forth to the court of Duchess Anne, the very young ruler of Brittany. She is to assist the girl by killing off those who would do her harm. But the world outside the convent is new and frightening to Ismae, who is used to the safety and sisterhood of Saint Mortain's. The person who scares her the most is Duval, Anne's half-brother. All the men Ismae has ever known have been nasty and brutish, but Duval is intelligent, gentle, and awfully interested in her.
Complex, romantic, and action-packed, this book will stay glued to your hands. I mean, seriously. Assassin nuns!