Eldunarí are like big crystal balls that house the souls of dragons whose bodies have died. The Big Bad has enslaved hundreds of them and allegedly destroyed the rest, making him practically unbeatable and the most powerful magician ever. But Eragon got a sort of prophecy early on regarding something called the Vault of Souls, and every time he asks about it the conversation goes much like this (paraphrased) one did on page 431:
Eragon: Could there be Eldunarí in the Vault of Souls?
Glaedr: Nope. Nuh-uh. No way. Not a chance. Don’t know what’s there, but it’s definitely not Eldunarí.
(About 130-ish pages later)
Heaps of Eldunarí: Psych! This Vault of Souls is actually full of souls! We totally had you going there! Right? Right? Oh, by the way, we’re responsible for every inexplicable turn of good fortune you have experienced during this whole series, and also here’s a portable pocket dimension to carry us all in so we can help you kill the villain! Wheeeee!
This plot “twist” was at least partially earned with set-up going all the way back to book one, so it’s not quite the biggest deus to ever have ex machina’d, but it’s pretty damn close. And great googly moogly, it is so clumsily done.