In the ending battle, Percy dies to save his friends. The others all accepted the offer of immortality, even Annabeth. His days in Elysium feel like an eternity in Tartarus, knowing he would never see them again. So, Percy, hurt by their betrayal, decides to take a swim in the River Lethe and be reborn. His only wish is that no one will know. Percico/ Perico/ Pernico/ Nicercy.