Grave Mercy (His Fair Assassin #1) by Robin LaFevers.
Ismae was born with a vivid red mark on her shoulder and back, apparently caused by an herbal concoction used in an abortion attempt. The herb-witch who provided the potion claims that the failure, and the mark left behind are signs that Ismae was sired by the god of death.
She's rescued by the local hedge-priest and the herb-witch, who spirit her off to the convent of St. Mortain, the patron saint of death, where young women purportedly sired by Death are trained to carry out his work, killing those with Death's mark on them. She settles in as a novitiate, learning not only to read and write, but basic anatomy, the "womanly arts", and how to fight both with and without weapons. She also becomes the apprentice of Sister Serafina, the poisons mistress.
But before her training is completed, she's given her first major assignment: she must play the part of a Viscount's mistress in order to learn who is trying to keep the young Duchess of Brittany from taking her throne.
This fascinating tale actually has some historical basis. There was a 12-year-old destined to rule the duchy of Brittany, whose ascent to the throne was complicated by multiple betrothals and other political intrigue.
The Sisters of St. Mortain are based on a group of druidesses called the Gallizenae, who lived on the Ile de Sein just off the coast of Brittany. They were Gallizenae said to possess mystical powers, healing, divination and controlling the weather and the tides, and possibly even the ability to fly.
At almost 550 pages, this hefty tome is a bit too long to finish in one night, but once inside Ismae's world it's difficult to leave.
Dark Triumph, the second book in the series, the story of Sybella, one of Ismae's fellow novitiates, is scheduled for release in the spring of 2013.
FTC Full Disclosure : I borrowed this book from my local library.