bộ the heroes of olympus 2: the son of neptune

In The Lost Hero, three demigods named Jason, Piper, and Leo made their first visit to Camp Half-Blood, where they inherited a blood-chilling quest: 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. Who are the other four mentioned in the prophesy? The answer may lie in another camp thousands of miles away, where a new camper has shown up and appears to be the son of Neptune, god of the sea.