So my wife is reading the series (at the end of HBP), and I was reading some stuff over her shoulder, and I found
more proof that
Harry's scar is a Horcrux. If anyone really wants me to go in depth on the theory, I will, but here's my latest discovery: Dumbledore theorizes that Voldemort used his murder of Frank Bryce to turn Nagini into a Horcrux. Therefore, we know from GoF that there is no visible difference between a Horcrux-creating AK curse and a normal AK curse (notice that JKR cleverly does not tell what Voldemort says when he kills Frank Bryce). From this we also learn that there is no special preparation done for the murder itself, or at least nothing more than a specific incantation. I'm guessing that in order to make an object a Horcrux, the more major magic would be done on the object. So I'd say that Voldemort whispered/thought whatever incantation he needed to prepare that piece of soul to become a Horcrux, and when the AK curse backfired, it melded a piece of his own soul to Harry, in the form of a lightning-shaped scar. But that last part we've gone over
A slightly more reaching part of that theory: Voldemort planned to use Harry himself as the final Horcrux, because it would be pure genius. Think about it: nobody is going to check the corpse of an infant for Horcruxes, and after being destroyed by such powerful magic, would Dumbledore himself be able to tell? He wasn't sure if the ring was a Horcrux, so I'm guessing a Horcruxed infant corpse would be indistinguishable from a non-Horcruxed infant corpse (forgive me if this is too coarse). The trophy itself is the grave of Voldemorts Final Challenger, in his own eyes, and when the corpse decays, the piece of soul lives forever in the soil. What would Dumbledore do, Voldemort imagines, destroy the whole world to destroy that one part of his soul?
