This is the 4th book in the Percy Jackson and The Olympians.
Percy attends freshman orientation when a mysterious mortal appears, Rahcel, followed by demon cheerleaders, everything goes from bad to worse. Time is running out as war between the Olympians and the evil Titan lord Kronos draws near. Even Camp Half Blood grows vulnerable. To stop Kronos from invading Percy and the other demi gods must set out on a quest through the Labyrinth, a sprawling underground world.