The Unforgivable Curses are three of the most powerful and sinister Dark Arts known to the wizarding world in the Harry Potter book series. They are tools of the Dark Arts and were first classified as "Unforgivable" in 1717. The three curses consist of the Killing Curse (Avada Kedavra), Cruciatus Curse (Crucio), and Imperius Curse (Imperio).
Although the use of any of these curses on another human being is considered grounds for life imprisonment in the wizarding prison of Azkaban, that doesn't stop Lord Voldemort and Death Eaters to use them in the bid to overthrow the Ministry of Magic and the rest of Wizarding World in both Wizarding Wars. In fact, this faction ultimately legalised their use after taking over the Ministry of Magic prior to their eventual downfall during the Battle of Hogwarts after which they eventually rendered illegal as they originally were.
There are 3 Unforgivable Curses in total, Imperius Curse, Cruciatus Curse, and Killing Curse.
Imperius Curse places the victim in a blissful trance or dreamlike state, leaving them under the complete control of the caster. The experience is described as a "wonderful release" from any sense of responsibility. The victim is calm (or rather in emotionless state), and capable of doing things they would be physically incapable of otherwise, to committing horrible crimes, and they obey any order as if the said orders are absolute. It is also seen that the curse works on animals, as Barty Crouch Jr. (disguised as Alastor Moody) casts the Imperius Curse on a spider to demonstrate it's effects to the Hogwarts students.
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, including Lucius Malfoy, also claimed to be under the Imperius Curse during the First Wizarding War to avoid imprisonment in Azkaban.
It is extremely difficult to overcome the effect of Imperius Curse, as only those with a particularly a strong will can do so. Harry Potter and Barty Crouch Sr. and Jr. each learned to resist the curse after being subjected to its effects. Harry Potter in particular appeared to be extraordinarily good at this, as he was able to partially resist the curse from the very first time it was ever cast upon him, and he was later able to completely resist it when it was cast upon him by Voldemort himself.
Cruciatus Curse 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. When Harry Potter was made the victim of the curse in June 1995, he described it as feeling like his head was being split open while having his bones set on fire. However, since this is the only time Harry is known to have been inflicted with the Curse, and as no one else has ever given a description of its effects, it is unknown if this is how the Curse always feels or if the experience changes depending on the victim.
Like other dark arts, malicious intentions are crucial elements required to cast Cruciatus Curse at its fullest. When motivated by anger rather than malicious sadism, the curse produces a brief, stunning burst of terrible pain, rather than the intense and sustained agony it usually causes. True to this, Harry was initially only able to send Bellatrix to her knees. Though he managed to achieve better success when casting it against Amycus Carrow, the pain he inflicted was only enough to knock him unconscious instead.
Cruciatus Curse was the very curse which caused Neville Long Bottom's parents Frank and Alice such suffering that they were admitted to the permanent ward at St Mungo's Hospital for Magical Maladies and Injuries as cases of insanity. This was carried out by: Barty Crouch Jr, and Bellatrix, Rodolphus and Rabastan Lestrange. As it was established that Cruciatus Curse being Bellatrix's signature spell, she may very well have used it against innumerable half-bloods, muggle-borns, and muggles just as Voldemort used Killing Curse against innumerable of them all.
Incantation: Avada Kedavra.
Killing Curse is a spell that causes a blindingly intense green bolt of light to shoot from the end of the caster's wand, which, on contact with the victim, results in instant 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 upon being struck. However, it seems to be more likely that the method of killing is due to the soul of the victim being magically ripped from their body. Evidence for this comes from the testimony of perhaps its most prolific caster; Lord Voldemort. Having experienced the lethal effects of a rebounded Killing Curse but surviving due to his creation of multiple Horcruxes, Voldemort described the sensation of being struck by it as having his consciousness torn from his body, accompanied by a sensation which he described as being "pain beyond pain". For Harry, on the other hand, he feels his sensation like the bruise of an iron-clad punch, in which the sensation wa likely have something to do with the curse that stripped part of Voldemort's soul in his body. Muggle coroners would be unable to find a cause of death in such an attack, but the Ministry of Magic recognizes the signs of the curse.
The Killing Curse has no counter-curse and cannot be blocked by magical means. However, the green energy bolt can be dodged or blocked with a solid object. If the curse misses the victim and strikes an inanimate object instead, it may cause a small explosion at the point of impact results, which may start a fire of green flame. However, the curse may also ricochet off the object in question.
Apart from phoenixes who simply reborn from their ashes as shown in case of Fawkes who sacrificed himself by swallowing a Killing Curse that Voldemort aimed at Dumbledore in 1996, only two beings that have ever been able to overcome the effects of this curse:
- Harry Potter, who was given magical protection against Lord Voldemort's use of the curse through his mother's loving sacrifice. 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 once 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 which amplifies his chance to return to life given that a part of Voldemort’s mutilated soul also resided on his body. This process somehow tethered Harry to life so long as Voldemort himself was alive.
- Voldemort himself due to his Horcruxes, which each have part of himself in them.
Being his very signature Unforgivable Curse Lord Voldemort murdered many people with the Killing Curse: his father, his paternal grandparents, James and Lily Potter, Charity Burbage, Frank Bryce, Gellert Grindelwald, Bertha Jorkins, and countless more with 6 of them done to create his Horcruxes (if not counted Harry as 7th Horcrux due to his conversion was pure accident as during the murder of James and Lily, Voldemort's soul was mutilated so much that a severed part of it struggles to kept attached on his main soul, only for Lily's sacrificial protection for baby Harry caused it ended up entered the baby's body).