Unforgivable Curses

The Unforgivable Curses are a group of three curses that are introduced in the fourth novel of the Harry Potter series (Harry Potter and the Goblet of Fire). They are the three most powerful and sinister Dark Arts spells known in the wizarding world, commonly performed by Dark wizards, such as Lord Voldemort and the Death Eaters. In theory, the usage of any one of these curses on another human instantly permits you a life sentence in the wizard prison Azkaban. In practice, Aurors have been allowed to use them during the First War, and the minitry closed his eyes about Harry's using of Crucio against Bellatrix Lestrange; Dolores Umbridge thought about using it against Harry, assuming she would be forgiven. Since the curses are very powerful, their use requires a strong desire and great skill to bring about the effects (making Harry's Crucio curse inneficient compared to Voldemort's one, noted Bellatrix).

The three Unforgivable Curses

 * 1) Avada Kedavra (The Killing Curse)
 * 2) Crucio (The Cruciatus Curse)
 * 3) Imperio (The Imperius Curse)

Avada Kedavra (The Killing Curse)

 * Description: One of the three "Unforgivable Curses". Causes instant death.
 * Seen/Mentioned: See below.

The Avada Kedavra Curse, Avada Kedavra, causes the caster's wand to shoot a bolt of blindingly intense green light, which, on contact with the victim(s), results in instant and painless death. There are no secondary effects; the victim simply "drops dead" for no biological reason. In Harry Potter and the Goblet of Fire, Muggle coroners examining the bodies of several victims are unable to find a cause of death, but the Ministry of Magic recognises the signs of the curse. If the curse misses the victim and strikes an object instead, the energy is released in a small explosion at the point of impact. If the material struck is flammable, the curse starts a fire composed of greenish flames that will burn the material to ashes within minutes. Phoenixes respond to being hit by Avada Kedavra the same way as any other means of death: they are consumed by fire, and are reborn from the ashes.

The Avada Kedavra curse is used regularly by Lord Voldemort and his Death Eaters to kill their enemies. The Avada Kedavra has no canonical counter-curse or blocking spell, although the caster can be interrupted, or the victim can dodge the green jet. Victims can also take cover behind solid objects, such as when Dumbledore Transfigures a statue to cover Harry in Harry Potter and the Order of the Phoenix. Certain magical conditions have also been documented to defeat the curse, even on a direct hit: Harry Potter was given magical protection against Lord Voldemort's use of the Curse, when his mother sacrificed herself to save him. Harry was also later saved by the Priori Incantatem effect between his wand and Lord Voldemort's during a duel; and Lord Voldemort himself was evidently saved from death by his own rebounding Avada Kedavra, because of his Horcruxes (although he was reduced at that moment to an ethereal disembodied state). In Harry Potter and the Order of the Phoenix, Fawkes sacrifices himself by swallowing a Killing Curse Voldemort aims at Dumbledore.

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 the Avada Kedavra or some other lethal curse.

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.

Crucio (The Cruciatus Curse)

 * Description: Inflicts excruciating pain on the recipient of the curse. Also known as the "Pain" or "Torture" Curse.
 * Seen/Mentioned: See below.

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 Bellatrix Lestrange) desire to cause pain purely for its own sake. The extreme pain inflicted by the curse when cast so - without any apparent evidence of physical harm - makes it uniquely suited as a form of torture, and is used regularly by the Death Eaters.

Dubious Uses
In Harry Potter and the Order of the Phoenix, Harry Potter used the Curse on Bellatrix Lestrange, in the Ministry of Magic Atrium, in vengeance for her murder of Sirius Black. The curse gave her a brief moment of terrible pain; it did not, however, cause her the excruciating drawn out pain that Cruciatus is meant to give.

Failed uses

 * In Harry Potter and the Order of the Phoenix, an enraged Dolores Umbridge attempts to use the incantation on Harry, but Hermione Granger interrupts her.
 * In Harry Potter and the Half-Blood Prince, Draco Malfoy tries to use the Cruciatus Curse during a duel with Harry, but was interrupted after Harry used the Sectumsempra spell against him.
 * In Harry Potter and the Half-Blood Prince, during the Battle Of The Tower and the Flight of the Prince, a Death Eater attempted this curse on Ginny Weasley. She dodged the curse easily, with the assistance of the Felix Felicis luck potion consumed earlier.
 * In Harry Potter and the Half-Blood Prince Harry Potter attempted this curse on Severus Snape. Snape blocks the curse and taunts Harry about his inability to use non-verbal spells and Occlumency. All Harry could say before being countered was the first syllable of the spell.

Imperio (The Imperius Curse)

 * Pronunciation: //
 * Description: Causes the recipient of the curse to do the unquestioned bidding of the caster.
 * Seen/Mentioned: See below.

The Imperius Curse places the subject in a trance or dream-like state, and leaves him or her subject to total control by the caster. The experience of being controlled by this curse is described as a complete, wonderful release from any sense of responsibility or worry over one's actions, at the price of one's free will. Resisting the effect of the curse is possible, however, and several individuals have been able to successfully stand against it, including Harry Potter, as well as both Bartemius Crouch Jr and Bartemius Crouch Sr, who learned to resist the curse after being subjected to its effects for an enduring period.

Failed uses
Herbert Chorley, a Muggle Junior Minister, who was the victim of a "poorly performed Imperius Curse." This resulted in his brains being addled, with the symptoms of him impersonating a duck in public. Harry Potter himself managed to resist on multiple occasions, once when the younger Crouch uses it on him, and again when Lord Voldemort attacks him with it.