The Last Olympian, the fifth and final book in the Percy Jackson series, was released on May 5, 2009.
Percy Jackson learns that Kronos' forces are preparing to attack Olympus. Poseidon, Percy's father, decides that it is time for Percy to now fulfill the Great Prophecy. Seeking to defeat Kronos, like Achilles did, Percy bathes in the River Styx, making his body invulnerable except one small chosen part of his body (the small of his back). Kronos leads a siege of New York City and puts its citizens to sleep. Percy leads the campers, Hunters, nature spirits, and centaurs to protect Mount Olympus from Kronos and his forces. While they protect Olympus, the gods hold down the monster Typhon as he makes his way to New York. Kronos, possessing Luke's body, forces his way into Olympus and battles Percy in Olympus' throne room. Typhon reaches New York but is defeated after the arrival of Poseidon's forces, led by Tyson. Annabeth is able to make Luke come back to his senses, and Percy gives him Annabeth's knife. Luke stabs himself in his mortal spot (as he also was invulnerable from bathing in the River Styx) to destroy Kronos and save Mount Olympus, but Luke dies. The gods reward Percy and his friends, and offer him immortality. He rejects the offer, but instead requests the gods to claim all their children and to have cabins for all the gods, including the minor ones. Rachel Elizabeth Dare becomes the Oracle and recites the next Great Prophecy. The book finishes with Percy and Annabeth becoming a couple, and ominous clouds looming over Rachel's next Great Prophecy.