The Gray Mage - Realm of Camellia Series, #3

To fulfill her bargain with the Storm King, Kira is finally going to go train at the Spire to learn more about her Shadow magic. There's just one problem: her Shadow magic doesn't work anymore. Something inside her soul is broken, and she must find a way to fix it, or risk losing her Light magic, too. As she struggles to cope with her inner turmoil, another battle is brewing outside the walls of the Spire when raiders descend on the realm.
And Kira's pretty sure they're there because of her. With her magic slipping and her newly discovered family's legacy at stake, Kira searches for answers. And a discovery about the past might hold some answers about the elusive Lord of Between. and an unpleasant connection with her broken soul.