The Blood Of Olympus (Heroes Of Olympus Book 5) by Rick Riordan
Seven heroic demigods. One final quest. The greatest sacrifice yet. ‘The ruins are up there,’ Piper promised. ‘I saw them in Katoptris’s blade. And you heard what Hazel said. ‘The biggest – ” ‘The biggest gathering of evil spirits I’ve ever sensed,” Jason recalled. ‘Yeah, sounds awesome.’ After surviving many deadly quests, Percy, Jason, Annabeth and their friends face one final challenge- defeat an army of giants before the Earth Mother, Gaia, finally awakes and brings about the end of the world . . .