The Heroes of Olympus

The Heroes of Olympus is a collection of five novels by American author Rick Riordan, scheduled to be completed in 2014.Riordan drew inspiration from Padraic Colum's The Golden Fleece and The Heroes Who Lived Before Achilles.The series chronicles the adventures of seven demigods as they race against time to prevent the awakening of the earth goddess, Gaea. The first novel, The Lost Hero, continues the story from The Last Olympian, which is the last novel of the preceding series, Percy Jackson & the Olympians. Percy Jackson & the Olympians is a series on the succeeding Gigantomachy of Greek mythology. A prophecy given in The Last Olympian outlines the events of The Heroes of Olympus. Demigods, living secretly in the modern era, are featured in both series. The Heroes of Olympus introduces Roman demigods: children produced by the Roman gods, who are separated from their Greek counterparts. These two groups of demigods are enemies due to a stolen statue of Athena and have been forced apart for centuries, but must unite to stop the main antagonist in the series, Gaea, from gaining control of the earth. Percy Jackson, the protagonist of the previous series, is again the main character.

Prophecy of Seven/New Great Prophecy

Known as the "New Great Prophecy" to the Greeks and the "Prophecy of Seven" to the Romans, this prophecy, introduced in The Last Olympian, guides the characters through the series. The prophecy reads as follows:
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.
The seven demigods the prophecy refers to are: Percy Jackson, son of Poseidon (Neptune). Annabeth Chase, daughter of Athena (Minerva). Jason Grace, son of Jupiter (Zeus). Piper McLean, daughter of Aphrodite (Venus). Leo Valdez, son of Hephaestus (Vulcan). Hazel Levesque, daughter of Pluto (Hades). Frank Zhang, son of Mars (Ares).
Much of the prophecy is clear. As of the end of The Mark of Athena, the identities of the seven demigods are known. Jason and/or Percy may represent the storm referred to in the prophecy. Leo is the self-styled "Wild Card", who is skilled in summoning fire and may therefore embody the reference to fire in the prophecy. There may also be a fire connection to Frank because his life depends on a burned stick, which can easily catch fire and kill him. The third line is relatively unsolved, but it could be about Annabeth's oath on the River Styx to do her best to protect Percy. This could lead to Annabeth getting captured or sacrificed, leading to Mars' statement about a sacrifice Percy cannot face. The fourth and last line of the verse about foes bearing arms to the Doors of Death simply means the Greeks and the Romans come together at the Doors. However, nobody knows for sure if they will work together, or fight, as Clarisse La Rue says.
Nevertheless, everything above should be considered speculation, as Riordan's prophecies normally do not make sense until they come to fruition. While most of the lines from the New Great Prophecy could be interpreted in the ways mentioned above, there is no telling for sure how the prophecy will be resolved.