Oathkeeper (Schooled in Magic Book 20)

The Allied Lands are in mortal danger. Three of the most powerful necromancers in the Blighted Lands have banded together into an unstoppable force, one capable of breaching the Craggy Mountains and laying waste to the lands beyond. Nothing will survive. And so Emily, the Necromancer's Bane, is called upon once more to stand in defence of her adopted homeland and stop the necromancers in their tracks.

But there is a complication. Six years ago, Emily made a promise to the Unseelie Court, swearing to give them whatever they wanted in exchange for their help. Now, that promise has been called in. The Unseelie want her to travel into the Blighted Lands, despite the risk, and carry out a mission for them. And if she refuses, she'll die.

Emily has no choice. She must take the war into the very heart of darkness itself ...

...And the enemy knows she's coming.