The Battle of the Labyrinth (Percy Jackson & the Olympians, Book 4)

After being attacked by empousai cheerleaders at his new school, Goode, Percy returns to Camp Half-Blood and learns about "The Labyrinth", part of the palace of King Minos in Crete that, according to Greek mythology, was designed by Daedalus. During a game at the camp, Annabeth and Percy find an entrance into the Labyrinth. Percy soon learns that Luke had used the entrance and will lead his army through the Labyrinth straight in to the heart of camp Half-Blood. To get into the Labyrinth, Percy has to find the Delta Symbol (Δ) on a passageway, touch it, and then enter the Labyrinth.