“The Heroes of Olympus” is centered around a prophecy, introduced in “The Last Olympian,” that predicted seven demigods would unite to protect the world from an awakening new enemy, the Earth goddess Gaea. Demigods from both the Greek camp, Camp Half-Blood, and a newfound Roman camp, Camp Jupiter, unite to save the world from being destroyed by Gaea. The prophecy reads:
"Seven half-bloods shall answer the call
To storm or fire, the world must fall.
An oath to keep with a final breath,
And foes bear arms to the Doors of Death."
Seven demigods—Annabeth Chase, Leo Valdez, Percy Jackson and Piper McLean from Camp Half-Blood, with Jason Grace, Frank Zhang, and Hazel Levesque from Camp Jupiter—join forces. Some other important characters are Nico di Angelo, son of Hades; Reyna Avila Ramirez-Arellano, Praetor of Camp Jupiter and daughter of Bellona; and Gleeson Hedge, a satyr.
