Bond of Blood Charm

• Type: Ancient Magic, Sacrificial Protection, Charm
• Creator: The specific application for Harry Potter was initiated by Lily Potter's sacrifice and subsequently extended and sealed by Albus Dumbledore.
• Basis: An act of willing, loving self-sacrifice by one person to save another. This is a form of deep magic that does not require an incantation or wand work.
• Effect: Creates a powerful, lingering magical protection over the saved individual. When anchored to a home shared with a blood relative of the sacrificed person (in this case, Lily Potter's blood living on in her sister, Petunia Dursley), it prevents the target's primary enemy from touching or harming them while they are within the confines of that home. The charm is fueled by love.
• Duration: The protection placed on Harry Potter at Number Four, Privet Drive was designed to last until he came of age at seventeen, or until he no longer considered the location his home, whichever came first.

The only known instance of this specific charm was the one used to protect Harry Potter. Its existence began on 31 October 1981, when Lily Potter sacrificed her own life to shield her infant son from Lord Voldemort in their home at Godric's Hollow. This act of sacrificial protection was so powerful that it caused Lord Voldemort's killing curse to rebound, destroying his physical body and leaving Harry with only a lightning-bolt scar. The initial effect of this magic was that Voldemort, in his spectral form, could not physically touch Harry. This was demonstrated when Professor Quirrell, who was sharing his body with Voldemort, was burned to ash upon making contact with Harry in the chambers beneath Hogwarts. Recognising the power of this ancient magic, Albus Dumbledore sought to strengthen and prolong it. He did this by placing Harry in the care of his only living blood relative, his aunt Petunia Dursley. By Petunia accepting Harry into her home, she sealed the powerful blood protection. As Dumbledore later explained, as long as Harry could call the place where his mother's blood dwelled “home,” Voldemort could not harm him there. This is why Harry had to return to the Dursleys on Privet Drive every summer, despite their mutual animosity. In 1995, Voldemort attempted to circumvent this protection. During his rebirthing ceremony in the Little Hangleton graveyard, he used Harry's blood, believing that by sharing the blood protection, he would be able to touch Harry without harm. While this did allow him to bypass the prohibition on physical contact, Dumbledore later revealed that Voldemort had made a grave error. By taking Lily's protection into his own veins, he tethered Harry to life, ensuring Harry could not be killed by him so long as Voldemort himself lived. The charm's power over the Dursleys' home was absolute. When Vernon Dursley attempted to throw Harry out of the house in the summer of 1995, Dumbledore sent a Howler to Petunia with the message, “Remember my last,” a reminder of the agreement she made to take Harry in and the protection it afforded him. The charm finally broke, as predicted, on Harry's seventeenth birthday, 31 July 1997. In anticipation of this, the Order of the Phoenix enacted a plan to move Harry from Privet Drive to a safe house a few days before his birthday, an operation that resulted in the Battle of the Seven Potters.

The Bond of Blood Charm is not a spell that can be learned or cast in a conventional manner. It is a manifestation of ancient magic that arises from a specific, profound act: pure, loving self-sacrifice. It is considered one of the most powerful forms of magical protection known. The primary limitation of the charm, as applied by Dumbledore, was its specific conditions:

• Location-Based: The full protection was only active while Harry Potter was at his home at Number Four, Privet Drive. While the base protection from Lily Potter's sacrifice always lingered in his blood, the impenetrable shield against Voldemort was tied to this location.
• Duration: The charm was temporary and set to expire when Harry reached the age of magical maturity (seventeen).
• Voluntary Anchor: The charm required Petunia Dursley's acceptance—however reluctant—of Harry into her home to be sealed. If she had refused him at the outset, or successfully thrown him out later, the charm would have broken.

There is no known counter-spell for this type of magic. Lord Voldemort's only recourse was to wait for the charm's conditions to expire, as he and his Death Eaters could not force their way into the Dursleys' home to attack Harry.

In interviews, J.K. Rowling has explained that the concept of a mother's sacrificial love being the most powerful magic is a central theme of the series. This ancient magic is what Voldemort, with his inability to understand love, consistently underestimates and fails to comprehend, which is ultimately a key to his downfall. In the film adaptations, while Professor Quirrell is still shown to be burned by Harry's touch in Harry Potter and the Philosopher's Stone, the detailed explanation of how Dumbledore extended this protection by tying it to the Dursleys' home is significantly reduced. The crucial conversation between Harry and Dumbledore at the end of Harry Potter and the Order of the Phoenix, where the charm is fully explained, is largely omitted from the film (film).