10 Things You Didn't Know About Harry Potter's Scar
2. It's Closely Linked To The Killing Curse
When Voldemort broke into Harry's family home on Halloween 1981, he murdered Lily and James Potter before turning his wand on their son. Casting Avada Kedavra - the Killing Curse - intending to murder the boy, the spell rebounded, sending Voldemort to the brink of death and giving Harry a lightning bolt just above his brow.
The scar is a direct result of being touched by the Killing Curse and surviving, but that might not be the only way the two are connected.
We already know why Rowling chose a lightning bolt, and we've mentioned the ancient rune that Harry's scar could represent, but why did the messy Avada Kedavra curse result in such a clean, noticeable shape? It might be because the hand movement used to cast the curse is exactly the same as the shape that Harry's scar took.
It's not clear if the whole "each spell has its own hand movement" thing is canon (this concept isn't really given much attention throughout the series, barring Wingardium Leviosa in The Philosopher's Stone), and in the movies, the spells had to look more cinematic, so that whole idea was pretty much dropped.
Still, it's interesting to think that the Killing Curse may have literally left its mark upon the one person able to survive it.