Hunting Prometheus: The Underworld Saga, Book 8 (Unabridged‪)‬

Hermie and Hestie never asked to be demigods. Unlike others of their kind, they aren't interested in going on quests and being the center of a world-changing prophecy. 

But when Zeus learns of Apollo's vision - of the twins finding Prometheus, Zeus's number-one enemy - the hunt for Prometheus begins. 

Zeus and his allies want to bind Prometheus and kill the twins before their threat of a revolution becomes a possibility. If Hermie and Hestie don't find the Titan first, they have no chance of fulfilling their destiny or of surviving Zeus's threat against them.