The Obsidian Tower by Melissa Caruso

The Obsidian Tower begins a bold new epic fantasy trilogy in which the broken magic of one woman will either save an entire continent-or completely destroy it.
As the granddaughter of a Witch Lord of Vaskandar, Ryx was destined for power and prestige. But a childhood illness left her with broken magic that drains the life from anything she touches, and Vaskandar has no place for a mage with unusable powers. So Ryx has resigned herself to an isolated life as the warden of Gloamingard, her grandmother’s castle.
At Gloamingard’s heart lies a black tower. Sealed by magic, it guards a dangerous secret that has been contained for thousands of years. Until one impetuous decision Ryx makes leaves her with blood on her hands-and unleashes a threat that could doom everything she loves to fall to darkness.