The Final Countdown
Remember Harry Potter and the Deathly Hallows? It wasn't Harry Potter and the Happy Ending—that would be another book entirely. When you make it to the end of a series, there has to be a sense of dread. And that, Shmoopers, is why we have The Blood of Olympus, and not, say, The Cookies 'n' Cream of Olympus.
But what does it mean for the plot? Well, when it comes to the Greeks and Romans, there's always a prophecy. This book is chock full of them, but the relevant one here is the one that says the blood of Olympus will awaken the Earth Mother, Gaea.
You'd think they could avoid the whole thing by staying as far away from Olympus as possible, but Percy and Annabeth, children of Poseidon and Athena, head right there and, of course, bleed all over the place. And while they're at it, they shout out the title:
The blood of Olympus, Piper thought with dread. (43.36)
Yep—it's the beginning of the end.