In percy Jackson the titans curse until the part that I have read, ,Percy and his firends recieve a mission based on a profecy and also they are tasked with liberating Athena who was captured by Luke and the General and they use her to lure the heroes into a trap and the evil forces have summoned undead soldiers who had transparent-grayish skin and they have military uniforms that are modern. There is a quote that is said when the prophecy is recieved and that shows the despair that awats the heroes of the camp since it says how they will loose people during the expedition. This shows the sacrifices that will be made during the expedition.
Five shall go west to the goddess in chains,One shall be lost in the land without rain,The bane of Olympus shows the trail,Campers and Hunters combined prevail p.89
This quote shows how there will be sacrifices during the mission and the prophecy is also true for when the hoeroes are in the desert they encounter a metal giant that was created by the titans and Bianca DiAngelo was lost in the desert for she stayed inside the metal giant when it ran away and they didnt see her again.