The Killing Curse is one of three 'Unforgivable Curses', curses considered to be such ruthless acts that the mere casting of one is a guaranteed trip to Azkaban prison. While the other two of the Unforgivable (the Imperius and Cruciatus Curses) are cruel enough (the former enables the caster complete mental control over the victim, while the latter inflicts a world of pain on the victim, often said to lead to mental instability), there is none so feared and distainded as the Killing Curse.

The Killing Curse, or Avada Kedavra, is of indeterminate origin. While the language of the incantation itself can be deciphered into a form of Aramaic (adhadda kedhabhra -'let the thing be destroyed'), it has been officially pronounced Doubtful that the alleged Jesus Christ and/or his disciples may have invented this curse. The incantation is also similar to the now-distinct incantation 'Abracadabra', which is said to be part of the Phonecian alphabet and not Aramaic. However, the histories of the Killing Curse and 'Abracadabra' (which is now little more than a nonsense word) can be traced to a similar source.

Many older wizards state that Avada Kedavra was once used as a cure for fevers and illnesses amongst magical peoples. The Gnostic wizard Serenus Sammonicus, a Potions Master, developed 'Abracadabra' as a protection against fever through the use of Abraxas stones (see Abracadabra Versus That Other One by Leviticus Damsel) with the incantation carved in an inverted triangle as follows:

However, it seems unlikely that either of these are a place of origin for the Killing Curse, the only known form of magic that kills upon striking and has no counter-curse.

Upon casting the Killing Curse, a violent green light is emitted from the wand of the caster. While the Killing Curse, like the Patronus, relies heavily upon the emotions of the wizard, the curse is as deadly as its name. It, like the other Unforgivable Curses, is completely irreversible once cast. The victim dies within an instant and is left with no trace of harm whatsoever; indeed, it is said that many Muggles have been mystified by the sudden deaths of family members who have appeared to have little wrong with them. Most often death by Killing Curse is related to stillness, a look of fear on the victim's face and of course the brightness of the green light.

As was previously mentioned, the Killing Curse relies upon emotion as well as concentration. It is not a curse to be attempted by novice wizards, nor can many elder wizards accomplish this spell. Of course, it would be highly illegal to train with this curse, though Aurors with the Ministry of Magic were trained on it during the reign of the Dark Lord. A main key to the Killing Curse is intent. While this would make it seem that those under the Imperius Curse could not cast it due to it not being their personal intent, if it is the intent of the original wizard, this rule does not apply. Mr Lucius Malfoy, for instance, was tragically forced to live under the Imperius Curse during the Dark Lord's reign in the late 1970s, 1980 and early 1981. However, for all of Mr Malfoy's noble intent, it was the intent of the wizard who cast the curse that caused his misfortune.

A wizard casting the Killing Curse must first master the intent to kill. While many wizards at one time or another have wistfully imagined throttling another, it takes fierce concentration to muster the true desire and intent to kill another. Of course, one can kill, for example, a Muggle that one knows little about and succeed despite having no long-standing history with the Muggle. This is the same sort of concentration required for the most rudimentary of spells; however, it is a deeper concentration that is difficult to master. Naturally, such a powerful curse takes the skill of an equally powerful wizard, and many wizards simply do not have the power to concentrate and balance their magic strongly enough to succeed in casting the Killing Curse.

While those less-powerful wizards may not succeed in killing someone with the Killing Curse, other physical damage has been known to occur due to novices attempting to cast this spell. Injuries have included nosebleeds, migraines and warts. (See So You Want to Go to Azkaban by Aurora Haversham)

There are other forms of magic that have been developed with the intent to kill; however all have a known counter-curse. The Draught of Living Death, for instance, a Potion so strong that it causes all who drink it to fall into a permanent period of sleep, has its own counter-potion. Amulets and charms which have been developed as deadly were later classified only as mildly annoying.

Wizards have worked to develop a counter-curse for the Killing Curse for many years; however, since there are few willing to participate in having the curse cast upon them to test the counter-curse, they have had little luck in finding a solution. As it stands, only those in the position of capturing Dark Wizards (Aurors and Unspeakables) are permitted to cast this curse without being sentenced to Azkabans. However, those working for the Ministry are advised to avoid casting this curse as much as possible, though one Auror, Alastor Moody, was said to use the Killing Curse at any given opportunity. Recently, Albus Dumbledore, Headmaster of Hogwarts School of Witchcraft and Wizardry, employed Moody as a Defence Against the Dark Arts professor in care of many children.

It should be noted that the Killing Curse has been survived by one child, Harry Potter, who was left so brutally disfigured that many say he may have been better off in death than to live a life so ugly. Potter (aka The Boy Who Lived), a Parseltongue like the Dark Lord, was a mere infant aged one year when he out-lived the curse. His parents, James and Lily Potter, died on the same Hallowe'en Night in Godric's Hollow that the Killing Curse was beaten for the first time. It was then that the Dark Lord disappeared, perhaps defeated by Harry Potter. However, in 1995 rumours of the Dark Lord's return began after Cedric Diggory (1977 - 1995) was killed during the re-birth of the TriWizard Tournament at Hogwarts School of Witchcraft and Wizardry. Diggory's death, unmistakeably caused by the Killing Curse, has been said to be the start of many. Sceptics, however, remain certain that the Dark Lord was forever defeated on that fateful night that Harry Potter was turned into a mutant.