The Titan's Curse by Rick Riordan

The Titan's Curse is the third book from the Percy Jackson series that starts with Percy, Annabeth and Thalia that are demigods the children of The Olympian gods plus Grover who is a , go to a military school to help Grover. When they arrive they realize that there are two powerful demigods in the school but when they try to sneak the kids out every thing goes wrong and they get attacked by a Manticore that was working in the school in human form. Then with the help of the goddess Artemis and her hunters they get the kids out safe but when they take the manticore down both Annabeth and the manticore fall of they hill and disappear. In the quest they realized about the The great stirring that will cause all of the biggest and the most dangerous monsters plus the Titans led by the evil Titan lord Kronos that have been sleep or considered dead or hiding will to rise and take over Olympus and rule the world. Than after everyone except the goddess go back to the camp with the help of Apollo the god of sun, the The Oracle of Delphi tells them that the goddess Artemis is in trouble and she has to be saved by a group of demigods and hunters "Five shall go west to the goddes in chains, One shall be lost in the land without rain, The bane of Olympus shows the trail, Campers and Hunters combined preval, The Titans's curse shall withstand, And one shall perish by a parent' hand". So 5 people are chosen, 3 Hunters and 2 Campers, and they have to go find the goddess. Percy that was not included in the group of the 5 has to find Annabeth and save her from trouble and try to stop the great stirring and Kronos.