Be Aware this is the fourth in the series meaning I can't help but have some spoilers in it! Seriously go start this series now! You won't regret it! If you are a fan of The Vampire Academy and Percy Jackson and the Olympians series that you will love the Covenant series!Apollyon is the fourth book in Covenant series and let me tell you it is by far the most explosive! This whole series has been building up to this book!At the end of Deity, Alex awakens as the Apollyon and turned to Seth, which was one of the most crazy cliff hangers I have ever read. As a main character I love Alex but she was so aggravating. "My Seth" was annoying and really funny. I was so elated when Alex went back to her normal self.Can Aiden get anymore wonderful? He stayed by her side when she was evil and beat him up! I love them as a couple and that grew throughout this book. They have such a great relationship and are perfect.I am so glad that there was even more use of Greek Mythology. Apollyon has Apollo playing a large role in this one too. We also met Persephone and Ares. I didn't quite expect Alex to go back to the Underworld that quick but the journey with Aiden was very exciting.Apollyon really demonstrated how much Alex has matured as a character throughout the series! Especially towards the end of the book when she learned her fate! I think she was able to deal with everything in this book a lot better. The old Alex would have either lashed out or turned away from everyone. I think this development of character was great to see. One thing that I loved about this series is that Alex wasn't always the strongest or most powerful she got defeated a lot! Yes she kicks ass though. The last fight scene was crazy Alex got her ass handed to her. It was semi emotionally disturbing!Two characters that are dead were brought back, though they are still dead obviously but this was done through interaction in the underworld where the dead live and help from the Gods! It was nice to see Caleb and Alex's mom. I like the idea that anyone can come back in this series in a ton of ways. So I had thought that Caleb’s death was one of the most emotional events in this series more so than Alex’s mom which is weird to say but for me I think Lea’s was worse. Alex may not have gotten the time to grieve yet which is why I think this will be harder for Alex in the next book but Lea was someone we just now in the fourth book got to fall in love with. Lea was always portrayed by Alex as this major bitch but we realize that she like Alex has gotten the short end of the stick a lot. In this series Lea has lost everyone she has ever loved so it was really just a sad thing to see this war take her life as well.I think that this book ended in an odd way. I felt that the ending would have been more impactful if it ended right after the huge fight scene. But it left the book on a lighter note. Though Apollyon was a very great and exciting book it was definitely more of a filler book. Several major things were revealed but compared to the rest of the series I don't think that much happened in this book.I am pretty sure that Sentinel is the fifth and last book in this series! I am going to be so sad to see it end! So much has to happen to tie up everything in this series! I don't doubt that Sentinel will be crazy and absolutely amazing!