"On past records I usually did start with a story or an idea for a song and then write around it, but on Achilles' Heel I would just start writing and try to let the song and my sub-conscience determine the direction. which is a goofy way of saying I tried not to decide before hand what the song and or the characters would do and be like."

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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