This was not what I expected at all, but I really enjoyed it anyway. I thought I was getting a fantasy/urban fantasy/mythology type book, but it really is historical fiction (set in the late 1400's in Brittany. which is apparently not Britain!).
Our heroine, Ismae, flees an arranged marriage at the age of 14, and enters a convent. But this is not a Christian convent, but one devoted to one of the ancient gods, Mortain, the god of death. Ismae was fathered by Mortain, and has some slight supernatural powers, but essentially is trained by the convent as an assassin, who specializes in poisons.
I thought the first part of the book, in the convent, was actually the slowest for me. Once she was out in the world practicing her craft, it really picked up. And personally, I would have liked more of the mythology/supernatural aspects to be played up. I did enjoy the court intrigue, and the very slowly building romance was pitch perfect.