My review contains possible spoilers of books 1-3Apollyon is the fourth book in the amazing Covenant series by the uber talented Jennifer L. Armentrout and is probably my favourite out of the series to date – a hard feat considering I have loved them all.JLA's Novella, Elixir (Covenant #3.5) can be read between these books to get an understanding of what happens when Aiden is forced to put Alex on the elixir, something which went completely against his beliefs and causes him a lot of pain and self loathing which shows in this book. {Find the novella for free here}The ending of Deity was heart wrenching, with Alex pulling away from Aiden and connecting with the First - Seth, Aiden was determined to find a way to break their bond but it didn't come without drama and consequences, the first few chapters were especially difficult as it just didn't seem it was possible but with determination we see some great changes.Alex is her ever snarky, self sacrificing, stubborn yet fiercely loyal self but in Apollyon we really start to see her embrace her destiny and to understand the power she holds, I think she has matured drastically since Half-blood – I absolutely adore her as a character.In this action-packed installment we see further plot progression, there is so much going on I was literally flying through the pages; without giving away spoilers - the gods have killed thousands in their quest to stop Seth from draining Alex of her power and killing off the gods. A lot was revealed and I was surprised by a few aspects and shocked by others, the villain was completely unexpected and left me gaping. The journey Alex undertook with Aiden was fraught with danger and despair and not everyone has come out of it unscathed.Seth – ugh! I knew that guy couldn't be trusted, he is power hungry and deceitful and I despise him that little bit more in each book I read! I am so curious to see how it all plays out in Sentinel, he didn't get as much page time in this book which I was quietly relieved about.The romance {sigh} I have cheered for Alex and Aiden's relationship from the very first book and to see their unconditional love and their devotion to one another despite the many obstacles standing in the way now out in the open was so sweet and wonderful, there were so many awww moments; I absolutely adore them together and at least the forbidden love aspect isn't as important as it once was now the world is in utter chaos.The ever-present, reliable secondary characters each played a supportive role to Alex and her plight and showed their devotion and loyalty especially Marcus, Olivia, Lea, Solos, Deacon, and Luke as well as a few surprise visits which was really nice to see (I won't mention by who).I loved reading about each of the gods, I was clueless about who to trust but they each played an important role in the overall story - JLA knows her mythology.The overall story progression has been phenomenal, the creative and wonderfully written story JLA has weaved has left me mesmerised and I'm actually quite sad the series is nearing its end; it has been a roller-coaster ride filled with all of the elements I really find entertaining in a Young Adult novel.Overall, JLA is an auto-buy for me and one of my all time favourite authors – Apollyon gave us the ideal mix of amazing characters, kick-butt action scenes, a beautiful love story, mythological intrigue and twists and turns filled with betrayals and secrets; It had me hooked from the very first to the last page and I literally sighed when I finished it.An exhilarating, captivating 5 star read – I can't wait for Sentinel to see how everything works out with the prophecy.