Percy Jackson and the Olympians is a book fandom. It is the first series of the Camp Half-Blood chronicles. The series follows the completion of the first Great Prophecy. The second Great Prophecy, later called the Prophecy of Seven that is revealed in The Last Olympian and is the main focus of The Heroes of Olympus, the sequel series.
THE GREAT PROPHECY
The series goes through the Great Prophecy:
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
This prophecy unravels throughout the series:
Percy Jackson was a half-blood child of Poseidon, one of the six eldest gods born of the Titan Rhea, wife of Kronos.
Percy reaches sixteen despite all of the numerous times he was almost killed by Luke/Kronos and his monsters.
Percy finds the whole city of Manhattan asleep from Olympus, so the war can go on without any of the humans interfering.
Luke Castellan used Annabeth's knife to kill himself to stop Kronos. This is the same knife that he gave her when she was little. He promised that they would be there for her and that they (including Thalia Grace) would be a family. When Luke broke his promise to Annabeth, her knife became a cursed blade.
Percy had to choose between giving Annabeth's knife to Luke and allow him to destroy himself or trying to destroy Luke himself. Either choice would have resulted in Luke's death however.
Percy's choice would result in the either the destruction of Olympus or its preservation.
The first three consecutive books of the series feature the revelation of a demigod child of the Big Three:
The Lightning Thief: Percy Jackson is claimed as a son of Poseidon.
The Sea of Monsters: Thalia Grace, daughter of Zeus, is restored to human form.