The Darkest Magic by Juliana Haygert

I’m losing my mind against the forces of dark magic… but it’s nothing compared to losing the vampire I love.
After experiencing horrible betrayal, I’m a prisoner to evil men who long to use me as a tool against the supernatural world. Against my will, I’m forced to serve dark forces for an evil that’s unimaginable.
The magic that’s overwhelming my body is causing me to go insane. I fall ill under the influence of wicked power, and those that are holding me captive are using my weakness as a trap for my beloved.
Killian will do whatever it takes to save me, but he’s running out of time. If we’re to stop those that seek to dominate all magical kind, he’ll have to set me free from my darkness.
It still might be too late. Once the darkness claims me, there’s no turning back.