Rating: 4/5 
 ***Minor spoilers & spoilers for books 1-3*** 
 Katy is trapped and lonely. The testing begins almost immediately after she arrives at the Daedalus compound. Before long, they have her pitted against other hybrids to test her abilities. Daemon is furious at having lost Katy, to the point where he's willing to do anything to get her back. Daemon goes to Luc, and Luc tells him that if he can get LH-11 (a substance that apparently cures disease) then he will get both him and Katy out of there. Daemon turns himself in under the condition that he gets to stay with Katy. Now they're biding their time for when they can get a hold on some of the LH-11 while also discovering some things that have been kept from them, such as just how powerful the child of a hybrid-Luxen pairing can be. 
 I think this was the best in the series so far, but it still hasn't risen high enough for me to give it more than 4 stars. This one is told in dual POV, which I found great at some points like when one knew something the other didn't or when they were apart, but at some parts of the book I found it a bit pointless. At times, the perspective changed quite randomly in the middle of a chapter and I didn't really see why. Besides that, my only other critique is the same one I usually have with this series, which is that the dialogue felt unnatural at times. Other than those two things, everything was fantastic. 
 I liked how much this book focused on Daemon and Katy's relationship. I know it bothered some people with how much they were together and how much it was about the romance, but their relationship is one of the most appealing things about the series to me. It was great how sweet they were together, and also seeing how far they're come as individuals and as a couple. Things got extremely intense by the end of the book, and a lot of stuff went down. There was action throughout, but there was a big battle at the end that was intriguing to read about. I thought the ending was good, but the very last chapter had me a bit confused. 
 Overall, I really enjoyed reading this and I think it was the best in the series so far. I loved seeing Daemon and Katy together more and more and seeing their character development. There was action scattered throughout the book but the end was where things got really exciting. I also found the introduction of the origins (children of Luxen-hybrid pairings) quite interesting. The last chapter was confusing but the rest of the ending made up for it. You should definitely pick this book up if you've finished book 3, and I'll be reading book 5 when I can.