The Last Olympian
It is revealed that Luke had bathed in the River Styx, thus becoming nearly invincible (much like Achilles). Seeking to defeat Kronos, Percy does the same, with some encouragement from Nico. It is also revealed how Nico's mother died, and that the Oracle's foretelling of her death led to Hades cursing the Oracle. Due to this, when Luke's mother attempts to become the Oracle, she loses her mind and is given glimpses of Luke's future. Rachel, a mortal who can see through the Mist, tells Percy that he is not the hero of the Great Prophecy and that it will influence his choice when he turns 16.