Blood Bond by Jessica Wayne

This magic wants her soul. And it will destroy anyone who gets in the way.

With Bronywyn’s light magic steadily dissipating at a rate she can’t begin to understand, it’s all she can do to hang on to the shred of humanity left in her soul. Now, the shadow magic is using her own haunted memories to drive her steadily insane.
In her lucid moments, Bronywyn knows it’s only a matter of time before the shadow succeeds—earning it a front-row seat to the rest of her life. Her only hope is that Tarnley has found a way to free himself from the magic binding them together before it’s too late to save them both.