|"Are you a wizard or not?"
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- "While you can still call home the place where your mother's blood dwells, there you cannot be touched or harmed by Voldemort. He shed her blood, but it lives on in you and her sister. Her blood became your refuge. You need return there only once a year, but as long as you can still call it home, there he cannot hurt you. Your aunt knows this. I explained what I had done in the letter I left, with you, on her doorstep. She knows that allowing you houseroom may well have kept you alive for the past fifteen years."
- —Dumbledore explaining how the charm works[src]
The bond of blood charm (incantation unknown) was an extremely powerful and ancient charm, which was formed when a person sacrificed themselves for a family member, out of deep and pure love. The sacrifice created a lingering protection in the blood of the person who was saved. It was not activated, however, until the charm was actually cast, and it was not sealed and functioning until another member of the family accepted the saved person as their own.
This powerful charm was cast by Albus Dumbledore on Harry Potter in 1981 after Lily Potter threw herself between Harry and the Darkest Wizard of all time Voldemort, when the latter tried to murder Harry. Lily refused to step aside and was brutally murdered, which made the Sacrificial Protection and allowed this charm to be cast.
The charm was ultimately sealed when Lily's blood-relative and sister Petunia Dursley took Harry into her house. This prevented Voldemort from hurting Harry until the day he turned seventeen, or when he could no longer call the house at Privet Drive his home.
- "She may have taken you grudgingly, furiously, unwillingly, bitterly, yet still she took you, and in doing so, she sealed the charm I placed upon you. Your mother's sacrifice made the bond of blood the strongest shield I could give you."
- —Dumbledore explaining the charm to Harry Potter[src]
If the victim was related to the beneficiary, then a powerful charm can be cast upon the beneficiary to give them additional protection (although it is unknown how closely the two must be related for this charm to work). This charm prevented any harm from coming to the beneficiary from the murderer while they were in a blood-related relative's home.
However, in order for this charm's power to take effect, the living blood-relative must first take the saved person into their home willingly. Once activated, this bond of blood will prevent harm from coming to the beneficiary when they are in their relative's home but it will break automatically when the saved person moves out permanently or turned seventeen (the age of majority in the British wizarding world), whichever happens first.
- Harry Potter and the Order of the Phoenix (Mentioned only)
- Harry Potter and the Deathly Hallows (Mentioned only)