SPOILER ALERT: This post continues “What Was Lily Potter’s Trick?” [and is continued in “A Bloody Better Theory“]. Many have speculated that Harry’s scar is the final horcrux, or hidden piece of Lord Voldemort’s soul. How Harry got it, however, remains mysterious. We know from the Dark Lord’s own mouth that he was trying to kill Harry, not create a horcrux:
“You all know that on the night I lost my powers and my body, I tried to kill him. His mother died in the attempt to save him — and unwittingly provided him with a protection I admit I had not foreseen. His mother left upon him the traces of her sacrifice. This is old magic. I should have remembered it. … I miscalculted my friends, I admit it.” (Goblet of Fire, Chapter 33.)
Dumbledore’s explanation is that love is the old magic, and Lily’s sacrifice of her life conferred protection on him. I’m not buying it. At least, that love is the full explanation. She pulled some trick, did some bit of magic, did some clever cross-up. Maybe it’s like this:
We know that Lily was good at potions and a favorite of Professor Slughorn, who spilled the secrets of creating horcruxes to Voldemort. Suggests, doesn’t it, that creating a horcrux involves potions? Further, we already know that creating a horcrux requires a killing.
The spilling of blood.
Recall that in the Goblet of Fire scene quoted above, Voldemort returns to life after taking a potion that contains Harry’s blood. Clearly, blood can be powerful medicine.
The books are full of references to “blood.” Mudbloods. Half-bloods. Pure bloods. Maybe it’s all a hint in a way.
Somehow, Lily Potter, using potion magic — and knowledge obtained perhaps from guilt-ridden potions master Slughorn — used her blood to make the decisive protective spell. If a horcrux is made by putting the blood of a victim upon an object, it had to be her blood, spilled or intentionally smeared on Harry’s forehead, that created the horcrux, creating the rebound effect that repelled the killing curse and saved his life.
It’s possible she used her husband’s blood, since he’d been killed first. Perhaps he killed James to create a horcrux. But before he put James’ blood on the object to stash some soul, she smeared some of her husband’s blood on Harry, making him the horcrux and protected. Kind of neat, except it doesn’t explain her need to sacrifice herself. No, the counter-charm for a horcrux-creating Avada Kedrava had to involve her own blood — and her death.
See how it fits? All the talk of blood? Snape, being the new potions master, and possibly in love with Lily Evans (later Potter), who came to his rescue once when James was ridiculing Snape? Snape might even have helped Lily devise this ultimate defense — which might explaining why he found it so galling he kept getting passed over as defense against the dark arts teacher.
In the final scene, the irony could be that Harry has to cut out his own scar — made from his mother’s blood — and shed his own blood in order to vanquish the One Who Must Not Be Named.
Afterthought: The horcrux potion would probably require Voldemort’s blood, too. After all, gotta be an agent of transfer, right? How Lily got that, I know of no hint. So maybe it’s not required at all.