In the Harry Potter books, an unforgivable curse is a curse which results in severe damage or death to the witch or wizard that it is performed on. If convicted of casting such a curse, the sentence is life imprisonment in Azkaban, the wizarding prison. Only three curses are considered unforgivable: the Cruciatus curse, the Imperius curse, and Avada Kedavra. Readers are introduced to these curses in Harry Potter and the Goblet of Fire during a Defense Against the Dark Arts class, when the curses are demonstrated on spiders.
The Cruciatus curse tortures the victim unmercifully. This unforgivable curse was commonly used by dark witches and wizards to extract information, and in some cases their victims were tortured to death or insanity. The parents of Neville Longbottom, one of the supporting characters in the books, lost their minds as a result of the use of this unforgivable curse. J.K. Rowling probably derived the name of the curse from the Latin word for “cross,” crux, which often appears in words meant to imply intense pain.
The unforgivable curse of the most concern to the Ministry of Magic is the Imperius curse, which is used to control the subject. An individual under the Imperius curse has no free will or ability to make decisions, acting essentially as a puppet of the dark witch or wizard who is casting the curse. Of course, a witch or wizard can also claim that he or she was under the Imperius curse when they carried out an act of treachery or violence, and the Ministry may find it difficult to prove otherwise. In Harry Potter and the Order of the Phoenix, this unforgivable curse is used on several characters with devastating effect. The name of this unforgivable curse is taken from the Latin root imperium, which means “to command.”
Avada Kedavra, the killing curse, is the most devastating unforgivable curse. When cast on a witch or wizard, he or she will die instantly in a rush of green light. As Professor Moody tells his students in Harry Potter and the Goblet of Fire, the wizard casting the curse must do it with intent and viciousness, or it will be ineffective. The name for this curse is likely taken from the word cadaver. There is no known counter curse, and the only person known to survive an attack is, of course, Harry Potter himself.