What was Voldemort doing in Godric’s Hollow the night Harry became an orphan? He was there specifically to kill Harry Potter, to prevent the fulfillment of a phrophecy he only half-heard. He succeeded in killing James and might have spared Lily – but she chose to sacrifice herself to save her son.
How did Lily Potter do it?
In The Goblet of Fire, it’s revealed that the curse Voldemort used to kill the Potters is the Avada Kedavra – or Killing Curse, the worst and deadliest of the three Unforgiveable Curses. It was widely known this was the spell Voldemort favored to kill his enemies. According to the imposter Mad-Eye Moody, it’s unblockable and without a countercurse. Let’s assume for a moment this information is accurate and use some reasoning: if the spell is unblockable, then it can’t ricochet and kill someone else, like a bystander or the spellcaster. So Lily didn’t have that option in defending her baby.
In The Order of The Phoenix, Dumbledore finally explains to Harry that the spell used to save him as an infant was an ancient blood-spell of protection cast by his dying mother.
So while it may be true the Avada Kedavra doesn’t have a precise counter, it can certainly be thwarted. In Harry’s case, it required the self-sacrifice of his mother. I surmise that Voldemort stood before Harry’s crib, preparing to cast the spell – and Lily deliberately threw herself in its path to save Harry while simultaneously casting the blood spell that would protect Harry.
But what would happen to the Avada Kedavra itself, once Lily cast her protection spell? It can’t be blocked. It can’t ricochet. Her wand and Voldemort’s did not share twin cores (according to Mr. Ollivander), so they wouldn’t nullify each other and cause the spells to work in reverse.
I contend the spell was transmuted – not quite death, but something very much like it. It managed to give Harry his scar, and bestowing onto Harry some of Voldemort’s skills – Pareltongue, an aptitude with Dark Arts, perhaps – and a shared mind-link. Then it ricoheted back to Voldemort, transforming him.
In The Sorceror’s Stone (or The Philosopher’s Stone, if you’re reading this in the U.K.) Voldemort reveals his shared body atop Quirrell’s head and says, “See what I have become? Mere shadow and vapor… I only have form when I share another’s body.” Further, Dumbledore says, later in that same chapter: “[Voldemort]… not being truly alive, he cannot be killed.”
What’s a name for an insubstantial thing that’s neither fully alive or quite dead?
A ghost that hungered to become a living thing again. Voldemort would spend the next thirteen years trying to become substantial again… by possessing Quirell, killing a unicorn, attempting to use the Stone and finally resurrecting his body in GoF.
But what of Harry, whose parents were just killed? It was Dumbledore who added the final protection: he gave him a charm that extended Lily’s blood sacrifice to Harry’s adopted home – his sister’s home. To quote Dumbledore in OoTP, “While you can still call home the place your mother’s blood dwells, there you cannot be touched or harmed by Voldemort. He shed her blood, but it lives on in her sister. Her blood became your refuge. You need return there once a year… Your aunt knows this.” [Italics mine.]
There is a statement, said by Voldemort and questioned by Harry, that has never been answered by Dumbledore. Harry first asked him in the first book, but Dumbledore declined to answer it then, and it never came up in .
Voldemort told Harry: “Your mother needn’t have died… she was trying to protect you…”
Why was Voldemort willing to spare Lily Evans Potter? What was so special about her that he would let her live when he killed so many others, including her husband?
Was it – love? Was Voldemort jealous of James?
Was it because he was trying to turn her?
Was it because of some other connection, relationship, obligation?
Did he promise her to Snape?
Here’s a wild guess: did Voldemort somehow know and love Petunia Evans and wanted to spare her the pain of killing her sister? Did Petunia struggle with her affection for the dark, forboding wizard and her love for the mystiphobe, Vernon Dursley? Is there some sick love triangle between He-Who-Must-Not-Be-Named and the Dursleys?
Ha Ha!. Okay. That last part’s a goof. I admit it.
Perhaps the reason why Voldemort was willing to spare Lily Potter is key to understanding the events of the next book.