This one wasn't my cup of tea. I'm going to go into some spoilers for some things that happen early on that set the tone for this book, so read at your own discretion.
This book starts with the main character having a form of amnesia. While I think that can work with a new series where you're learning along with the characters, in this one, we have all of that backstory, so it was more a retelling of things that had already occurred and we knew about already.
Also the setting is primarily in some tunnels that felt drab and morose. My final complaint is that the characters don't seem to be growing in terms of personal growth. Sure, they're uncovering new powers, but the relationships and the way they interact with each other are the same in book 6 as they were in book 1. If there is change in their personalities, it's more grim and dark. Hence, not my particular cup of tea.
This entire review has been hidden because of spoilers.