The Nevernight Chronicles by Jay Kristoff

Mia Corvere is only ten years old when she is given her first lesson in death.

Destined to destroy empires, the child raised in shadows made a promise on the day she lost everything: to avenge herself on those that shattered her world.

But the chance to strike against such powerful enemies will be fleeting, and Mia must become a weapon without equal. Before she seeks vengeance, she must seek training among the infamous assassins of the Red Church of Itreya.