Sacrificial protection

"You see, when dear sweet Lily Potter gave her life for her only son... she provided the ultimate protection. I could not touch him. It was old magic."

- Voldemort on Lily Potter's sacrifice

Sacrificial protection is an ancient, powerful, and lasting magical protection that is created when one person sacrifices his or her own life willingly and out of love to save the life of one or more people.

Effects
Sacrificial protection can be conferred on a single person or on a group of people. In cases involving a single person, the protection prevents whoever murdered the person who sacrificed their life from physically touching the person saved, and will cause a Killing Curse cast at the saved person by the murderer to rebound. In cases involving multiple people, the extent of the protection is not known, but it seems that it is less than in single-person cases, so that spells cast by the murderer at those under the protection will simply wear off more quickly rather than be reflected back (although it is unknown how that would apply to the Killing Curse, as there is no gray area with that curse's effects).

It is unknown if anyone employed by the offending witch or wizard can harm anyone protected by the sacrifice. This is seemingly evidenced during the Battle of Hogwarts, where it is believed that Harry protected his friends from Voldemort, after which, not even any Death Eaters, much less Voldemort himself, was able to do any damage to the Hogwarts army. However, the possibility of coincidence still remains, particularly since Lucious Malfoy was able to shake Harry's hand in 1992, despite him being a loyal servant of Voldemort.

Bond of blood


If the person who sacrificed their life was related to the person they saved, then a powerful charm can be cast upon the saved person to give them additional protection. However, in order for this charm's power to take effect, another living blood-relative must first take the saved person into their home. Once activated, this bond of blood will prevent harm from coming to the saved person when they are in their relative's home, but it will break automatically when the saved person moves out permanently or turns seventeen (the age of majority in the British wizarding world), whichever happens first. If the relative takes the protected wizard in, they are not allowed to kick that person out against his or her will until they become of wizarding age, even with good reason (although it is never explained what the consequences are of kicking them out anyway, as, when confronted with that fact Petunia Dursley simply stated that Harry had to stay).

Known occurrences

 * On the night of October 31, 1981, Lily Potter sacrificed her life in order to protect her infant son, Harry, from Lord Voldemort. This placed Harry under magical protection, so that when Voldemort cast the Killing Curse at Harry in turn, the spell backfired, leaving Harry unharmed (save for a scar on his forehead) and Voldemort bodiless. Harry became the only known person to survive the Killing Curse because of the power of his mother's loving sacrifice. However, forseeing Voldemort's inevitable return, Albus Dumbledore furthered Harry's protection by casting the above-mentioned charm on him and leaving him in the care of his only living relative, Petunia Dursley, Lily's older sister.
 * On May 2, 1998, during the Battle of Hogwarts, Harry Potter willingly allowed himself to be hit with a Killing Curse cast by Lord Voldemort, so that the piece of Voldemort's soul residing inside him would be destroyed and the Dark Wizard could be defeated once and for all. This conferred a certain amount of protection on Harry's allies, making it so that spells Voldemort tried to place on them, including Silencing Charms and a Full Body-Bind Curse, quickly wore off.

Appearances

 * Harry Potter and the Philosopher's Stone
 * Harry Potter and the Philosopher's Stone (film)
 * Harry Potter and the Goblet of Fire
 * Harry Potter and the Goblet of Fire (film)
 * Harry Potter and the Goblet of Fire (video game)
 * Harry Potter and the Order of the Phoenix
 * Harry Potter and the Deathly Hallows