Here's what you're getting into. K, so this book picks up from Elixir. Side note: Definitely read Elixir before you read Apollyon otherwise you'll have no clue what they are talking about. Alright, so Alex is completely psycho and off her rocker and Aiden needs her to break the connection with Seth before Alex breaks his heart. And while Alex is completely MIA because of this creepy connection, the whole world is literally falling apart. So eventually in a very touching scene, Alex breaks the bond with Seth and they are on the road to figuring out how to solve the 2-Apollyon situation. And that road leads to none other than Hades' Kingdom: The Underworld. This was pretty intense and of course we get to catch up with Caleb, a fan favorite. And then things get a little dicey down there with some newly acquired information and then you're reading and you're like: oh, crap, things are never going to be the same ever again. 
 Oh, and every time I thought I'd figured something out....NOPE. 
 Don't even try to guess what is going to happen in this book because it will literally BLOW YOUR MIND. 
 I have no idea how this is all going to pull together for Sentinel. There is so much at stake! And I feel emotionally tied to every character in this book. I'm going to died if there are anymore casualties. Which brings me to Sentinel. THE LAST BOOK. Just one question: HOW AM I GOING TO SURVIVE ONCE THIS SERIES IS OVER?! No, seriously. This is a real problem. THE STRUGGLE IS REAL. 
 Once again, I have to point out how much I LOVE Alex. She just gets better with each book. She is the sass master of all time. I bestow on you the Gold medal of sass. 
 Deacon could be runner-up to the sass master title. He had a bunch of good one-liners in there. I love him even more this book. I probably has to do with all the brotherly interaction between him and Aiden. I love the St. Delphi brothers. 
 Which brings me to Aiden. MY LOVE. My heart is fluttering in my chest and I'm finding it hard to breathe. Seriously, can he be anymore amazing? He just keeps getting better and better. And I'm pretty sure some of Alex's sass is rubbing off on him...and I like it. 
 And Apollo? He's growing on me. LOLZ with his impeccable timing.