Grave Mercy: His Fair Assassins Series by Robin Lafevers
Ismae is the daughter of the God of Death. When she finds her way to the convent of St, Mortain, she becomes a handmaiden of Death. A carefully trained assassin. Her first assignment brings her into a world of political intrigue. This world makes her question everything she was taught at the convent, leading her to an impossible decision. His Fair Assassin's series follows three novitiates as they find their way among the powerful of medieval Brittany.