Ismae has escaped from the brutality of an arranged marriage into the sanctuary of the convent of St. Mortain, the god of Death. Here, she learns that he has blessed her with both dangerous gifts and a violent destiny. If she stays, she'll be trained as an assassin, one of Death's handmaidens. To claim this new live, she has to destroy others. Her most important mission takes her to the high court of Brittany where she finds herself under-prepared, not only for the deadly game of intrigue and treason, but the choice she must make. How can she deliver Death's vengeance upon a target that has stolen her heart?Grave Mercy was dark and mysterious in a number of ways. The convent and St. Mortain, the old buildings and keeps, Ismae and Duval. This is a book that sucked me into the historical setting and the dangerous battle of life and death, who lives and who dies.The best compliment I can give is that this book felt historical, that the author nailed Ismae's voice and the Brittany setting. It felt dark and dreary, like a castle of old with gaps between the stones in the walls letting cold air in, candles giving off light after the sun went down, dirt roads becoming mud in the rain underneath the horses' hooves. Everything felt dark and dismal and hopeless, a perfect backdrop for the book. And Ismae was wary and cautious, but still willing to carry out her duties, blending into the background as women often did, listening for secrets that might reveal her mission. She was still battered and bruised from her life before the convent, and so she had to move carefully to get past it.There was a lot of intrigue, of suspicion and finger-pointing. I was never sure who was on which side or who to trust until the very end.The romance was woven into the intrigue nicely, where the result of the former largely relied on the result of the latter. It never felt forced, Ismae was often conflicted around him, but I imagine that's more to do with the horrible relationship with her father and her almost marriage. She is a handmaiden of Death, she she fear no one, but in her heart, she does fear this one man.The one slight drawback that I've noticed in historical novels is how often the characters plan out events and how often they are kept waiting. Some spots dragged as Ismae waited for news from the convent, as she waited to discover which character would turn to the other side. Of course, that is all do with the time period itself, it did take longer to send information from one person to another. And some of that time was filled with Ismae spending her days and nights at lavish parties with odious people she was forced to associate with.This book is definitely a must-read for historical fans who don't mind a bit of otherworldly mystery and romance thrown in. Also for people drawn in by this simple description of the book: assassin nuns in 15th century Brittany.