The Last Olympian, the fifth and final book in the Percy Jackson series, was released on May 5, 2009.
Percy Jackson learns Kronos' forces are preparing to attack Olympus. Seeking to defeat Kronos, Percy bathes in the River Styx, to make his body mostly invulnerable. 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 are holding off Typhon as he makes his way to New York. Kronos, possessing Luke's body, forces his way into Olympus and he and Percy battle in Olympus' throne room. Typhon reaches New York but is defeated. Luke is able to overcome Kronos, and Annabeth, realizing the meaning of the great prophecy, tells Percy to give Luke her knife. Percy does, and Luke stabs himself in his mortal spot to destroy Kronos and save Mt. Olympus. The gods reward Percy and his friends, they offer him immortality but he refuses, but they grant him one request. He requests the gods to claim all their children. Rachel becomes the oracle and reads the next great prophecy. At the end Percy and Annabeth make their relationship official.