The Last Olympian (Percy Jackson & the Olympians, Book 5)

Percy Jackson, his friends, and the Olympians fight in a war resembling the original war between the Greek gods and the Titans and in a final battle with the powerful Titan, Kronos. They ultimately defeat Kronos in the battle and Olympus is saved. Many fight for Olympus, including the Hunters of Artemis, dryads, satyrs, naiads, and tree nymphs, Chiron's centaur cousins the Party Ponies, and the hellhound Mrs. O'Leary. They have to defend Mt. Olympus while defeating Kronos' huge army consisting of rogue half-bloods, dracaenae, Hyperboreans, Laestrygonians, a drakon, Hyperion, telekhines, and hellhounds.