From Burning Ashes (Collector Series #4‪)‬

This war is not being played on a battlefield.

Killed by the man she loves, not even death is kind to Zoey. Neither life nor her fae powers will let her go in peace.

In this final installment, Zoey’s strength, determination, and trust are put to the ultimate test. In the fight to protect the people she loves and stop DMG from experimenting on fae, humans, and animals to create super soldiers, she learns the reality of how far Dr. Rapava will go to achieve his ultimate mission.

Of course, Rapava is not the only one willing to go to extremes. The demon, Vadik, is hunting Ryker, and there is nothing he won’t do to get back the property he feels is his: both Ryker and the Stone of Destiny.

To end one evil, Zoey has to make a deal with another. The most powerful demon in the world, the unseelie King.
And it might be the deadliest decision of all.