Great Prophecy

The Great Prophecy was a prediction made by the Oracle of Delphi that foretold the rise or fall of Olympus by a child of the Big Three.

The Great Prophecy
The First Great Prophecy was a prophecy given by the Oracle over seventy years ago. Around World War II, when the sons of Zeus (America), Poseidon (Great Britain and Soviet Union), and Hades (Nazi Germany, Fascist Italy, and Imperial Japan) fought each other, the Oracle foretold of a future event that would end their affairs with mortal women for the next seventy years. The prophecy is revealed in The Last Olympian:

A half-blood of the eldest gods

Shall reach sixteen against all odds

And see the world in endless sleep

The hero's soul, cursed blade shall reap

A single choice shall end his days

Olympus to preserve or raze

Unraveling the Prophecy

 * 1) Percy is a half-blood child of Poseidon, one of the eldest gods.
 * 2) Percy turns 16 at the climax of the Battle of Manhattan, despite all his near death experiences.
 * 3) Morpheus, god of dreams, put every mortal in Manhattan to sleep.
 * 4) "The hero" does not refer to Percy, but to Luke, whose soul was reaped by Annabeth's knife, cursed because of Luke's broken promise to Annabeth.
 * 5) Percy's choice was to give Luke the knife allowing Luke to kill himself and therefore killing Kronos, ending his life.
 * 6) Olympus was preserved because Percy chose to give Luke Annabeth's knife so that Luke could destroy Kronos by cutting his Achilles' heel (under his arm).

The Prophecy of Seven
After the completion of the first Great Prophecy, Rachel Elizabeth Dare took on the spirit of the Oracle of Delphi and spoke a new prophecy, which was declared as the second Great Prophecy. This prophecy would later be known as the Prophecy of Seven as the Romans called it.

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 first line of the prophecy refer to these seven demigods:

1) Jason Grace : A roman demigod; son of Jupiter

The Sea of Monsters
The Great Prophecy is told to Percy by the the Oracle. She tells him that when a child of eldest gods reaches 20 against all odds, the prophecy will come true. Throughout the movie, it is implied that the cursed blade is Riptide.

Trivia

 * The Great Prophecy never says that a child of the Big Three would be the one mentioned in the prophecy, only that it would be a child of the eldest gods. Because 'gods' can be considered a gender neutral term in some cases, this could mean that any demigod child of the original Olympians could be the one mentioned in the prophecy, including children of Demeter (Hera and Hestia as well, if they had demigod children).
 * However the term 'gods' is usually attributed to the male gods, which is why only the children of Zeus, Poseidon, and Hades are the focus of the prophecy.