Muggles' Guide to Harry Potter/Magic/Imperio
|Muggles' Guide to Harry Potter - Magic|
|First Appearance||Harry Potter and the Goblet of Fire|
The Imperius Curse forces the victim to obey the caster's commands.
Extended Description 
Unlike the Killing Curse, it is possible to fight this curse. These two and the Cruciatus Curse are considered to be the most terrible curses in the magical world, called the Unforgivable Curses; the use of any of the three on another witch or wizard is punishable by a life sentence in Azkaban.
Only very skilled wizards are able to fight this curse, and only very powerful ones are able to cast it – it takes significant mental strength to impose your will upon another. Interestingly, Harry Potter seems to have an innate ability to fight the curse, while Ron Weasley is more susceptible to it than average. The spell is first seen and described in the fourth book, and the effects on Harry and Ron are described in the next chapter.
When called upon to name the three Unforgivable Curses, it is Ron who comes up with this one. Professor Moody comments that this curse had made things difficult for the Ministry, as one could never be sure whether the person you had arrested was a true Death Eater, or was doing the will of a Death Eater who had remained hidden.
We see a number of people controlled by means of the Imperius curse throughout the series in and after the fourth book. Harry himself has need to cast this curse in Harry Potter and the Deathly Hallows. The description that we receive at the time, of the action of the curse on the caster, lead us to believe that while a wand is necessary to cast this curse, the actions are somewhat longer-lasting; Harry, for instance, casts this spell on the goblin Bogrod, then on the Death Eater Travers, and has them both under his control at the same time. One gets the feeling that it is only necessary to actually have the wand actively engaged with the subject of the curse while he is getting his instructions; as is seen at the end of that episode, Travers has been instructed to hide himself, and he continues to serenely do so even after Harry has gone.
Greater Picture 
The list of people we know who are controlled this way is quite large.
In Harry Potter and the Goblet of Fire, we learn that initially Barty Crouch is being controlled by his father, Bartemius. Then, Bartemius is being controlled by Wormtail, while Alastor Moody is being controlled by Barty. There is also a stretch where, possibly, Bertha Jorkins is being controlled by either Wormtail or Voldemort. Voldemort tries to control Harry with this curse, and fails. It is perhaps ironic that Harry's ability to throw off the curse was nurtured by Barty Crouch, who claims to be Voldemort's greatest supporter.
In Harry Potter and the Order of the Phoenix, we believe that Sturgis Podmore was trying to get through a door at the Ministry because he was under this curse, and we are led to believe that Broderick Bode was acting under Lucius Malfoy's instruction when he went mad. Harry guesses that Umbridge may be controlled in this manner, but Sirius says that is probably not the case.
We see relatively few instances of the curse in Harry Potter and the Half-Blood Prince. The story is littered with techniques meant to detect people under the Imperius curse or disguised by other means, which seem marginally effective; in fact, when we first see Tonks on the Hogwarts Express, we are almost led to believe that she is under the Imperius curse. There is an interesting case of Imperius-by-proxy as well; Draco Malfoy admits at the end of the book that he has placed Madam Rosmerta under the Imperius curse, and it seems that while under control, Madam Rosmerta put Katie Bell under the Imperius curse as well.
In Harry Potter and the Deathly Hallows, we see two people under the Imperius curse in the first five chapters: Pius Thicknesse, and Stan Shunpike. Once the Ministry falls, we can safely assume there will be many others, but they don't directly affect the story, except for the goblin Bogrod and the Death Eater Travers, as mentioned above.