When Luke had descended into the River Styx, he would've had to focus on something important that would hold him to his mortal life. Otherwise he would've dissolved. I had seen Annabeth, and I had a feeling he had too. He had pictured that scene Hestia showed me—of himself in the good old days with Thalia and Annabeth, when he promised they would be a family. Hurting Annabeth in battle had shocked him into remembering that promise. It had allowed his mortal conscience to take over again, and defeat Kronos. His weak spot—his Achilles heel—had saved us all

Rick Riordan