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