Unforgivable Curses

Unforgivable Curses are three of the most powerful and sinister spells known to the wizarding world in Harry Potter 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).

Imperius Curse
Imperius Curse a spell which enable the user to use mind control over their victims. The spell’s name is Imperio.

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 calm (or rather in emotionless state), from 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 he also casts the Imperius Curse on a spider during class.

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.

Countermeasures
Resisting the Imperius Curse is possible, but extremely difficult. Only those of a 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. 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
Is the curse that enable the user to inflict cruciating pain on the victims. The spell’s incantation is Crucio.

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. Like other dark arts, malicious intentions also required in order to successfully cast this curse: Though proved to be able to cast the curse, Harry Potter proved to be more incompetent than other Dark Wizard as he mostly cast it out of righteous wrath. 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.

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 as cases of insanity. This was carried out by: Barty Crouch Jr, Bellatrix Lestrange, Rodolphus Lestrange, and Rabastan Lestrange.

Killing Curse
The incantation of Avada Kedavra 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 the 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 it's 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.. 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.

Lord Voldemort murdered many people with this curse: his father, his grandfather, his grandmother, James Potter, Lily Potter, Charity Burbage, Frank Bryce, Gellert Grindelwald, Bertha Jorkins, and likely many more with 6 of them done to create his Horcruxes (if not counted Harry as 7th Horcrux due to his conversion was pure accident).

In 1996, Fawkes sacrificed himself by swallowing a Killing Curse that Voldemort aimed at Dumbledore. It killed him as it would anything living, but in a typical display of a Phoenix's response to death, he burst into flames and was immediately reborn from his own ashes.

Only two people 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.

Countermeasures
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.