I don't know how anyone alive could resist that premise, frankly.  I am in thrall, and this is my favourite of the three books that make up Robin LaFevers' trilogy.

The setting is a medieval, alternate-history Brittany, just after a young Anne becomes duchess.  An older, nine-god religion (based on Celtic deities) fights for primacy with the new Christianity.  (Religious geekery.  I love it.)  The assassin nuns are the instruments of Mortain, the God of Death, and are deployed according to his wishes by the Abbess of the convent.

Dark Triumph is the second of the series, and it is dark.  Very dark.  It doesn't hold back on cruelty or abuse. The main character, Sybella, is sent back into the family situation that nearly drove her into madness in the first book, Grave Mercy.  I thought my own family was dysfunctional, but hers beats mine into a cocked hat.  The story follows her as she navigates the dangerous waters of her suspicious, manipulative father, over-possessive brother, ladies-in-waiting who spy on her every move, and over-eager retainers, all the time attempting to follow the orders of her convent and keeping her real mission secret.  

Sybella thinks of herself as something dark, almost evil.  The story's romance features a man who sees the good in everyone, Sybella included, and the way she finally comes to accept herself and her place in the world is a treat to read.  

In so many books featuring strong women, the heroine still ends up rescued by the man.  Not in this book.  I also love the real friendship between Sybella, Ismae (the heroine of Grave Mercy) and Annith (Mortal Heart), the relationship between the daughters of the convent and Anne of Brittany, and the complicated, sore relationship between the nuns and the Abbess - who may not be all she seems.