Nevernight / Jay Kristoff.

From New York Times bestselling author, Jay Kristoff, comes a dangerous new fantasy world and a heroine edged in darkness. Destined to destroy empires, Mia Corvere is only ten years old when she is given her first lesson in death. Six years later, the child raised in shadows takes her first steps towards keeping the promise she made on the day she lost everything. But the chance to strike against such powerful enemies will be fleeting, so if she is to have her revenge, Mia must become a weapon without equal. She must prove herself against the deadliest of friends and enemies, and survive the tutelage of murderers, liars and daemons at the heart of a murder cult. The Red Church is no ordinary school, but Mia is no ordinary student. The shadows love her. And they drink her fear.