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 RiordanSon of Poseidon? East asked. I nodded. Took a dip in the Styx? Hudson asked. Yep. They made digusted sounds. Well that's perfect East said. Now how do we kille him?
Rick RiordanHe touched the screen as if trying to reach through with his hand. "You're a wonderful young lady. I don't tell you that often enough. You remind me so much of your mother. She'd be proud. And Grandpa Tom" --he chuckled-- "he always said you'd be the most powerful voice in our family. You're going to outshine me some day, you know. They're going to remember me as Piper McLean's father, and that's the best legacy I can imagine.
Rick Riordan