Killing Curse

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Avada Kedavra (Av-ada Ked-av-ra) is the incantation name for the Killing Curse, one of the three Unforgivable Curses in the Harry Potter book and film series. The curse is mentioned in the first novel, but not actually seen until the fourth Harry Potter novel when Barty Crouch Jr. (disguised as Alastor Moody peformed it in a classroom at Hogwarts on a spider.

Appearence and Effect
The Killing Curse appears as a jet of green light, which, when it comes in contact with the target, results in the target's instant and painless death. Voldemort is known to have used the Killing Curse frequently. There is no known counter-curse for the spell; the only known defense against it are dodging the spell, hiding behind a physical barrier (which is destroyed upon impact), creating Horcruxes, and the voluntary self-sacrifice of a relative, which causes ancient magic to protect the intended victim from the curse.

Survivors
Only one person is known to have survived the curse: the young Harry Potter was saved when his mother, Lily Potter, decided to block the curse herself instead of having her son killed; later on, when he let himself be killed by voldemort, the curse, which was conjured by Elder Wand that is loyal to Harry, had only leave Harry unconscious while destroying the bit of Voldermort's soul within Harry. When the curse used on Harry rebounded, Voldemort's body was destroyed, but, owing to his many Horcruxes, his spirit was unable to leave the physical world, and was later placed in a new body identical to his old one. The Phoenix Fawkes could also survive The Killing Curse. He swallowed one intended for Dumbledore, burst into flames, and was reborn from the ashes. It is debatable whether Voldemort could be said to survive The Killing Curse, as it backfired on him after hitting Harry, leaving him nearly dead but still alive.

Etymology
Avada Kedavra may ultimately come from the ancient Aramaic language, unlike Rowling's other spells, which derive from Latin. Avada Kedavra may come from the conjurer's word "abracadabra" (from Aramaic avra kedavra "I create as I speak"). Rowling may have taken her spell from a form such as avara kedavra or "I destroy as I speak", which differs from the "I create" form in just one letter.

Failed uses

 * Voldemort attempted to use the curse on Harry Potter as an infant - the curse was blocked by magical protection from his mother's sacrifice and somehow gave Harry some powers. Not only that, the curse backfired on Voldemort who barely survived.
 * Lucius Malfoy attempted to use the curse on Harry Potter or Dobby outside the Headmaster's Office, at the end of the film version of Chamber of Secrets . Malfoy pointed his wand at the pair and muttered "Avada...", but he was interrupted by Dobby with some kind of charm in the form of a ball of fire which threw Malfoy backwards. In the book version, Malfoy "lunged" at Harry, and was blocked by Dobby.
 * Voldemort attempted to use the curse on Harry Potter during their duel in the graveyard, in Goblet of Fire . Harry's simultaneous Expelliarmus charm blocked Voldemort's curse and produced the Priori Incantatem effect, due to the common core in their respective wands (phoenix feathers from Fawkes).
 * Death Eater Antonin Dolohov attempted to use the curse on Hermione Granger towards the end of Order of the Phoenix but was disarmed in time by Harry Potter.
 * Voldemort attempted to use the curse on Harry Potter towards the end of Order of the Phoenix but was blocked by Dumbledore who transfigured a golden statue at the fountain in the atrium of the Ministry. In the ensuing duel between Dumbledore and Voldemort, Voldemort attempted to use it several times against Dumbledore but was blocked and resisted successfully each time. This is possibly due to Dumbledore's skill and his use of elder Wand, which makes its user undefeatable in duels.
 * Vincent Crabbe attempted to use the spell on Hermione Granger and Ron Weasley in Harry Potter and the Deathly Hallows but the spell missed them both.
 * Voldemort used the curse on Harry Potter near the Climax of Harry Potter and the Deathly Hallows, However Harry Survived, due to Voldemort Using Harry's blood for his recreated body, so the protection Harry had from his mothers death was also given to Voldemort, So Harry could not die while Voldemort was alive.

Disputed uses

 * Bellatrix Lestrange killed a fox with a non-verbal spell after Apparating with Narcissa Malfoy at Spinner's End, thinking that it was a lurking Auror, but it is unknown whether it was the Avada Kedavra or some other lethal curse.
 * Sirius Black and Remus Lupin thought about killing Wormtail lauching a spell together but were stopped by Harry. It is unknown whether the spell used would have been Avada Kedavra.
 * Bellatrix Lestrange hit Sirius Black with a spell that made him fall into The Veil. It is not stated if the curse she used was Avada Kedavra, so it is disputed if it was the curse that killed him or him falling into the veil. However, in the film versionof the Order of the Phoenix, Bellatrix Lestrange is shown using the curse and spoke the incantation, leading people to believe it was the killing curse.