MIKK
Lily Potter killed Voldemort the first time. She created a blood oath that stated that if Voldemort kills her than he can't kill Harry. And by subsequently killing her he thereby agreed to that oath. So then when Voldemort tried to kill Harry his magic backfired shattering his Hocruxed soul causing some of it to be imbedded into Harry's scar.
Writing style is cool but your characters so far have had no personality besides what is required to have a story if you could put something more in their perspective to allow us to understand their actions and reactions that would make them more human and understandable or more unlikable, thanks just some friendly criticism from experienced reader.