The last Olympian beginning middle and end

Kronos, who has reincarnated in the body of Percy's old friend, Luke, has sworn an oath of vengeance to destroy Olympus, brick by brick. He has been massing an army of monsters, demigod children, demons, and a few other Titans for a year.

In order to stand against an overwhelming army and to fight Kronos, Percy accepts the offer of his friend Nico, son of Hades, and bathes in the river Styx. After emerging from the river, his body immortal except for one small point at the small of his back, Percy leads the charge in defense of Manhattan. After a series of brutal battles against overwhelming odds, after slaying several mythical monsters, Percy, Annabeth, and Grover catch up to Kronos in the throne room of the main palace at Olympus. After fighting Kronos for a while, Annabeth is injured and Percy is disarmed. Kronos, noticing the way Percy is effected by her injury, maneuvers between the two demigods, and threatens to kill her should Percy continue.

Percy, who led the defenders of Olympus and ultimately saved the gods, is granted one wish. The gods offer him immortality and a place as one of them for eternity. He turns this down in order to stay mortal and live his life with Annabeth. Instead of becoming a god, Percy makes the gods swear on the Styx that they will do a better job of claiming their children and getting them to Camp Half-blood, as well as recognizing every god including the minor ones at Olympus.