The Imperius Curse is one of the Unforgivable Curses. When cast successfully, it places the victim completely under the caster's control, though a person with exceptional strength of will is capable of resisting it. This makes it unique among the unforgivable curses, as it is the only one of them with a known, reliable (depending on the victim's will) method of resistance. The curse's incantation is Imperio.
Unlike the other Unforgivables, being subjected to the Imperius Curse - when adequately cast - is not an unpleasant experience, in fact, quite the opposite; the victim of an Imperius Curse is placed in a calm, trance-like state in which all feeling of responsibility and anxiety is banished. An adequately Imperiused being is placed under the caster's total control and may be directed to do anything the caster wishes, including crimes such as murder, political corruption, embezzlement, and so on. Also, whilst under the caster's control, the curse may also endow the victim with whatever skills that are required in order to complete the task at hand, such as increased strength or allowing them to cast spells far above their level.
If the Imperius Curse is performed poorly, then the victim would have their mind addled. It seems the damage is long-lasting, being sent a mental hospital because of the magicalmMaladies and injuries to recuperate. This is likely due to the fact powerful, dark magic may leave irreversible biological damage, such as when George had his ear cursed off permanently. Resisting the Imperius Curse is possible, but requires great strength of will and character. Resisting the Imperius Curse is similar to Occlumency, which requires a great amount of willpower, though it is unknown if Occlumency would actually work against the curse. It is possible for someone who has been Imperiused to place others under the curse as well.
When a correctly cast Imperius curse is terminated - for whatever reason - the victims become themselves again, as happened after the final defeat of Voldemort and the Death Eaters in the Battle of Hogwarts in 1998, when all of the people across the country who had been Imperiused by him or his supporters were released from the spell. Also, any pain that the curse has negated by the pleasant sensation would return, along with any other pain that the victim suffered for the duration of the curse.
The Imperius Curse was invented during the early middle ages by dark witches or wizards. The curse was created for the coercion and brainwashing of others into slavery.
After the Wizards' Council was reformed into the Ministry of Magic, tighter restrictions were placed on the use of certain kinds of magic. The Imperius Curse was deemed by the Ministry to be dark magic, and, along with the Cruciatus and Killing curses, were declared "unforgivable" in 1717. The use of any of these three curses on a fellow human being would result in a life sentence in Azkaban, unless there is sufficient evidence that the caster did so under the influence of another's Imperius Curse.
During the First Wizarding War, the three Unforgivable Curses were legalised for Aurors against the Death Eaters in order to win the war, fighting violence with violence. This was repealed once the war was over, as it was no longer necessary. While claims of being under the imperius curse were accepted, the Ministry has gone as far as to assign several personnel to determine who is truly under the curse, and who is lying to escape punishment; however, they were not perfect in capturing deception, as Draco Malfoy along with several others, still eluded justice.
When Lord Voldemort took over the Ministry, the three curses were once again legalised: this time every wizard and witch had the right to use them as they please. In fact, they were practiced in Hogwarts as part of the curriculum in the Dark Arts class under the tutelage of Professor Amycus Carrow. After Voldemort's death and the revolutionising of the Ministry under Minister Kingsley Shacklebolt, the three curses were once again forbidden, and many people under its influence felt like they were coming out of trances.