Apollyon (Covenant #4) by Jennifer Armentrout!!

Fate isn’t something to mess with… and now, neither is Alex. Alex has always feared two things: losing herself in the Awakening and being placed on the Elixir. But love has always been stronger than Fate, and Aiden St. Delphi is willing to make war on the gods—and Alex herself—to bring her back. The gods have killed thousands and could destroy entire cities in their quest to stop Seth from taking Alex’s power and becoming the all-powerful God Killer. But breaking Alex’s connection to Seth isn’t the only problem. There are a few pesky little loopholes in the whole “an Apollyon can’t be killed” theory, and the only person who might know how to stop the destruction has been dead for centuries. Finding their way past the barriers that guard the Underworld, searching for one soul among countless millions, and then somehow returning will be hard enough. Alex might be able to keep Seth from becoming the God Killer… or she might become the God Killer herself.

My take
First of all, I'm extremely biased since Jennifer is, in my eyes, a goddess of stories! Therefore, all I can say is Apollyon is all we've waited for and MORE!

The story picks up a few days after Elixir ends, with Alex and Aiden at her cell. Does Alex reconnect to Seth or is she strong enough to fight him? Will Apollo destroy Alex to prevent her from connecting with Seth? Can Seth be destroyed? Obviously, I'm not gonna tell you :) I find it very challenging to give an honest review without spoilers.

I will say, Jennifer has outdone herself and ups the ante with Apollyon by answering some our questions from the previous books while creating new questions (If you haven't already, I very highly recommend you read Deity and Elixir prior to Apollyon.). I was so desperate that I emailed Jennifer with my questions; she simply replied that my questions will be answered in Sentinel (shocking response, eh?).

Wow...the characters! You can see the them mature and grow in different ways.  Out of the confines and structure of the covenant, some begin to "loosen up" - as much as you can in a dangerous atmosphere.  I switched to Team Aiden in Elixir, and Apollyon just reinforces why we love him! He's just as protective and stubborn as ever though a softer side is shown. Seth is still Seth but in a different capacity.

The cliffhanger will have you sitting on the edge of your seat, gasping for more! Or in my case, emailing the author!