J.R. Ward 
Jackal 

The #1 New York Times bestselling author of The Sinner brings another hot adventure of true love and ultimate sacrifice in the Black Dagger Brotherhood world.The location of the glymera’s notorious prison camp was lost after the raids. When a freak accident provides Nyx clues to where her sister may still be doing time, she becomes determined to find the secret subterranean labyrinth. Embarking on a journey under the earth, she learns a terrible truthand meets a male who changes everything, forever. The Jackal has been in the camp for so long he cannot recall anything of the freedom he once knew. Trapped by circumstances out of his control, he helps Nyx because he cannot help himself. After she discovers what happened to her sister, getting her back out becomes a deadly mission for them both. United by a passion they can’t deny, they work together on an escape plan for Nyxeven though their destiny is to be forever apart. And as the Black Dagger Brotherhood is called upon for help, and Rhage discovers he has a half-brother who’s falsely imprisoned, a devious warden plots the deaths of them alleven the Brothers.