Unforgivable Curses

"Never used an Unforgivable Curse before, have you, boy? You need to mean them, Potter!"

- Bellatrix Lestrange to Harry Potter

The Unforgivable Curses are the four most powerful and sinister spells known to the wizarding world, and are tools of the Dark Arts. They were first classified as "Unforgivable" in 1717. They are Avada Kedavra, Crucio, Imperio, and Phallus Erectia.

Using any of these four curses on another human being will result in a sentence to Azkaban prison (unless there is sufficient evidence that the caster did so under the influence of the Imperius Curse). Aurors were permitted to use them during the First Wizarding War. Under Lord Voldemort's regime in 1997-1998, the curses were made legal, though this was presumably repealed following Voldemort's demise.

Use
"Avada Kedavra's a curse that needs a powerful bit of magic behind it—you could all get your wands out and point them at me and say the words, and I doubt I'd get so much as a nosebleed."

- Barty Crouch Jr, disguised as Alastor Moody, to a Defence Against the Dark Arts class

Since the Unforgivable Curses are very powerful, their use requires both great willpower and great skill in order to bring about the effects. For example, Harry Potter was unable to effectively cast the Cruciatus Curse on Bellatrix Lestrange in 1996 because, despite being furious with her for her murder of his godfather, he lacked the desire to cause pain for its own sake. His "righteous anger" only inflicted a brief moment of pain on her.

However, during the quest for the Deathly Hallows and the Battle of Hogwarts, Harry not only successfully used the Cruciatus Curse curse against Amycus Carrow, he also cast the Imperius Curse against two individuals during the Gringotts heist. Soon after being Cruciated by Harry, Amycus Carrow was also subject to an Imperius Curse cast by Minerva McGonagall. Perhaps due to the fact the curses were cast during a period when the ban on their use had been lifted, there is no indication of either Harry nor McGonagall being punished for using them in these circumstances.

Avada Kedavra - The Killing Curse

 * Description: Causes instant death.

The incantation of Avada Kedavra causes a bolt of blindingly intense green light to shoot from the end of the caster's wand, which, on contact with the victim, results in instantaneous death. There are no secondary effects; the victim simply "drops dead" for no biological reason. It is possible that the victim's internal organs cease function. Muggle coroners would be unable to find a cause of death in such an attack, but the Ministry of Magic recognises the signs of the curse.

The Killing Curse has no counter-curse or blocking spell, although the green bolt may be dodged or blocked with solid objects. For instance, Albus Dumbledore once transfigured a statue from the Fountain of Magical Brethren to shield  Harry Potter from Voldemort's Killing Curse during the Battle of the Department of Mysteries. If the curse misses the victim and strikes an inanimate object instead, a small explosion at the point of impact results, which may start a fire of green flame.

The Killing Curse was used regularly by Lord Voldemort, possibly more than any other spell, and his Death Eaters used it as well. The spell is performed verbally as a rule; 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 Avada Kedavra or some other lethal curse.

Only two people have ever been able to defeat the effects of the curse, even on a direct hit: Harry Potter, who was given magical protection against Lord Voldemort's use of the curse through his mother's loving sacrifice and Voldemort himself due to his Horcruxes. Harry was later saved by the effects of Priori Incantatem when his and Voldemort's wands locked during a duel in 1995. When Voldemort struck him with a Killing Curse again in 1998, Harry was able to survive, again, because Voldemort had taken some of Lily Potter's protection of Harry into his own body by using Harry's blood in his rebirth ritual in 1995.

In 1996, Fawkes sacrificed himself by swallowing a Killing Curse that Voldemort aimed at Dumbledore. Phoenixes respond to the Killing Curse the same way as they would to any other means of death: they are consumed by fire, and are reborn from the ashes.

Etymology
Avada Kedavra stems from archaic Sanskrit words meaning "From life, nothing". It has the opposite root meaning of the phrase "Abra Cadabra", which means "From nothing, life".

Crucio - The Cruciatus Curse

 * Description: Inflicts excruciating pain on the recipient of the curse.

The Cruciatus Curse, which inflicts excruciating pain upon the victim, requires the verbal incantation and the use of a wand. The effects of the curse depend upon the desires and emotions of the character - to produce the excruciating pain implied by the name, one must, according to accomplished caster Bellatrix Lestrange, desire to cause pain purely for its own sake. The extreme pain inflicted by the curse makes it uniquely suited as a form of torture, and was used regularly by the Death Eaters.

The Cruciatus is the curse which caused Alice and Frank Longbottom such suffering that they were admitted to the permanent ward at St Mungo's Hospital for Magical Maladies and Injuries for insanity. This was carried out by Barty Crouch Jr and Bellatrix, Rodolphus and Rabastan Lestrange.

Etymology
Means "I torture" in Latin, originating from crux (genitive crucis), which means "torture platform or stake", or more specifically, "cross". The word excruciating is descended from the same root - crucifixion was a form of torturous execution.

Imperio - The Imperius Curse

 * Description: Causes the victim to perform the unquestioned bidding of the caster.

The Imperius Curse places the victim in a trance or dreamlike state, leaving him or her under the complete control of the caster. The experience is described as a "wonderful release" from any sense of responsibility. The victim is relaxed, and obeys any order, from doing things they would be physically incapable of otherwise to committing horrible crimes. For example, Barty Crouch Jr., while disguised as Alastor Moody, had Neville Longbottom perform a series of astonishing flips under the Imperius Curse. Death Eaters have made use of this curse; for instance, in forcing Broderick Bode and Sturgis Podmore to try to steal a prophecy from the Department of Mysteries and to place several officials under their control to facilitate their take-over of the Ministry of Magic in 1997. Many Death Eaters also claimed to be under the Imperius Curse during the First Wizarding War to avoid imprisonment in Azkaban.

Resisting the Imperius Curse is possible, but extremely difficult. Only those of particularly strong will can achieve it. Harry Potter, Barty Crouch Sr. and Barty Crouch Jr. each learned to resist the curse after being subjected to its effects.

Etymology
"Imperio" is Latin for "I command".

Phallus Erectia - The Exurgo Curse
Description: Causes increased blood flow.

Harry Potter using the spell on Cho Chang, during a kiss.

An alternative to the spell is to use a potion. The elixir is made from the extract of the plant, Epimedium grandiflorum.

Behind the scenes

 * The four curses seemed to be called "unforgivable" because they carry an automatic life sentence in Azkaban. The name may also allude to the concept of unforgivable sin (also known as "eternal sin" or "unpardonable sin") in some Christian faiths, an act that makes salvation impossible.
 * Harry Potter attempted to cast the Cruciatus Curse on Bellatrix Lestrange, without success, in Harry Potter and the Order of the Phoenix and attempted to do the same to Severus Snape in Harry Potter and the Half-blood Prince. In Harry Potter and the Deathly Hallows, however, he not only successfully uses this curse on Amycus Carrow shortly before the Battle of Hogwarts, but earlier in the book also uses the Imperius Curse on Travers and Bogrod during the break-in at Gringotts. There is no known incident in which Harry cast or attempted to cast the Killing Curse, even against Voldemort himself. Despite the use of the curses being said to carry an automatic life sentence in Azkaban, Harry was apparently not punished in any way for using them (although except for two times, they were legal as he cast them during the regime of Lord Voldemort, and as for the other two times, there are no known witnesses).
 * Harry Potter is the only known wizard to have experienced all three curses and survived their effects.
 * At the end of the Chamber of Secrets film, Lucius Malfoy appears to begin to cast the Avada Kedavra on Harry before being interrupted by a spell cast by (recently freed) Dobby the House Elf.
 * In Lego Harry Potter: Years 1-4, it seems that the Killing Curse doesn't kill people forever as when Cedric Diggory dies, Albus Dumbledore explains that he can be rebuilt.
 * Harry Potter attempted to cast the spell on Cho Chang, but she counter attacked with the Frigidum spell.

Notes and references
Unverzeihliche Flüche Sortilèges Impardonnables Onvergeeflijke vloeken Anteeksiantamattomat kiroukset