Review: 
 Protagonists: With so many heroes on this quest I had thought that there wouldn't be enough story to really give each one enough presence in the adventure. I was sort of right, while many characters had their "shining moments" where each one grows and develops even the slightest amount, I still felt like some of the more featured characters could have given up a moment or two so that more overshadowed characters could get a bit more of the spotlight. That being said I did get enough of each character so that I'm not ranting and raving that Rick Riordan isn't giving "my favorite character(s)" enough "screen time." 
 Romance: One of the things I love most about Rick's books is that since they're written for an MG audience there isn't much romance. Don't get me wrong I do love reading a good romance in a book from time to time, but the way that Rick incorporates the romance in his books seems natural and nearly real. Like many of his other books, this one has fairly little love and romance, sure there a few cute romantic moments between Percy and Annabeth in Tartarus, but other than that there are really only three chapters that have any real semblance of growing romance, and the main star of those chapters is Leo. Yes, the kind-of sort-of love triangle between Leo, Hazel, and Frank will be resolved, but it happens in a very unexpected and unconventional way. 
 World-Building: You would think that after eight books and many short stories in this world that there wouldn't be anymore world-building. I mean this world is fairly thoroughly fleshed out, but I forgot about the one thing we've never really seen before, Tartarus. Yes, the deepest depths of the underworld where many evil monsters call home until they find away to crawl and slither back up to the earth is finally revealed. There was a lot I had expected to see from this adventure and what I got was everything I wanted and so much more. In addition to Titans, Giants, and monsters we also get to see a few primordial beings who like Gaea existed since the beginning of time, and of course there's always the fairly human mythological baddies who made a deal with Gaea to return to the world of the living and are causing our heroes trouble. 
 Predictability: Rick Riordan ceases to amaze me with the twists he puts into his books. There's a fairly big shocker involving fan favorite Nico di Angelo, which I called just a few pages before it was revealed but even so it was still fairly shocking and I loved this twist. It gives his character depth, not like he was really lacking any, but even so it is something that may spark controversy, though I hope it doesn't, and I am interested to see where the author goes with this, or if it's something that's there, accepted, but not really explored. The few other big twists did shock me and even if I saw something from a mile away, the author's great storytelling capabilities made the entire adventure so memorable regardless of what I did and did not foresee. 
 Ending: The ending definitely sets up the ending of this amazing spin-off series. There are choices made and allies who left, and I could practically see it all leading up to something amazing. The actual final sentences were mellowed and not an agonizing cliffhanger like we got last time, but you can see the storm clouds on the horizon and it's all pretty much stating that the next and final adventure for The Heroes of Olympus is going to be huge, action packed, and possibly tragic as the end draws near and the lines of The Prophecy of Seven will come to pass. 
 Rating: 
 Okay, so this rating should really come as no surprise to those who know how much I love Rick Riordan, and while my rating may be a bit biased I would be the first person to tell you if he wrote a bad book, and this one is amazing. Considering that there isn't really a formal, prophecy laden, quest for our heroes to embark on, unless you count the Prophecy of Seven but that's more of a prophecy that encompasses the series and not this single installment, this is a wonderful addition to this series. Sure here and there it feels a bit like filler, but it set up the story and our heroes before their final confrontation.