Percy Jackson - The Last Olympian

In the beginning of book five, Percy and Charles Beckendorf, son of Hephaestus, are sent on a quest to blow up Luke's monster cruise ship, the Princess Andromeda. However, Kronos knows of the plan, and captures Beckendorf and Percy. Beckendorf then sacrafices himself to save Percy. Percy feels guilty about leaving Beckendorf, although Nico, the son of Hades, says he saw him in the Elysian Fields, and was told to tell Percy not to blame himself. Percy and Nico sneak out of camp, and meet Hestia, who shows Percy part of Luke's past, after they meet Luke's mother, who tried to become the Oracle, and was thus driven insane by a curse of Hades' working. Luke has bathed in the River Styx to become invulnerable, allowing him to survive the presence of the Titan Lord.

Percy travels with Nico to the Underworld, where he bathes in the River Styx despite warnings from the Achilles himself, thus becoming invulnerable.

As he returns to the surface, a silent Manhattan is assaulted by several armies. Percy leads forty demigods in a defense effort, activating the many statues Daedalus has planted throughout Manhattan for more manpower. He is also joined by the Hunters of Artemis and many nature spirits under Grover's direction. Even with these forces, Percy cannot hold back the tide of monsters, and is slowly forced backwards to the Empire State Building.
Annabeth saves Percy's life by getting in the way when Ethan Nakamura tries to stab him in his one weak spot, the small of his back. Annabeth is seriously hurt and while she is recovering, Percy reveals the location of his "mortal point," or Achilles' heel, to her.
Luke is injured fatally and frees himself of Kronos second-in-command leadership, and as his life seeps away, Annabeth admits she never loved him except as a brother. Thus completing the prophecy. As a result of his role in winning the war against the Titans, Zeus offers to make Percy a god, but Percy declines, asking instead that the pact against the "Big Three" having children be ended, and for all the gods and goddesses to pay more attention to their demigod children, and requests that they claim them by the time they are thirteen.
Percy and Annabeth kiss in the dining pavilion after Annabeth says, "I am never going to make things easy for you, Seaweed Brain. Get used to it." The other campers catch them in the act and playfully toss them into the canoe lake, where Percy creates an air bubble, and then they have what Percy describes as "pretty much the best underwater kiss ever.