Red Sister (Book of the Ancestor, Book 1) by Mark Lawrence
It's not until you're broken that you find your sharpest edge
I was born for killing - the gods made me to ruin
At the Convent of Sweet Mercy young girls are raised to be killers. In a few the old bloods show, gifting talents rarely seen since the tribes beached their ships on Abeth. Sweet Mercy hones its novices' skills to deadly effect: it takes ten years to educate a Red Sister in the ways of blade and fist.
But even the mistresses of sword and shadow don't truly understand what they have purchased when Nona Grey is brought to their halls as a bloodstained child of eight, falsely accused of murder: guilty of worse.