Unforgivable Curses

The Unforgivable Curses are three curses in particular that when used on another human being can result in a life term in Azkaban. These three curses were used extensively by Voldemort’s followers during his rise to power in the 1970s, and their use by Aurors against suspects was in turn authorized by Bartemius Crouch Sr at that time. One, the Killing Curse, was used by Voldemort himself on Harry Potter, but the curse backfired and Voldemort was defeated.

The three curses are:

• Cruciatus Curse (“Crucio”)
• Imperius Curse (“Imperio”)
• Killing Curse (“Avada Kedavra”)

References from the canon

• These three curses have been "Unforgivable" in Wizarding society since 1717 according to Dumbledore (TBB)
• Fake Mad Eye Moody (Barty Crouch Jr.) performed each Unforgivable Curse on a spider in front of the class. He said the penalty for using an Unforgivable on another human being is life imprisonment in Azkaban (GF14).
• During the Battle of the Department of Mysteries, Harry tried to use the Cruciatus Curse on Bellatrix Lestrange, but it didn't have much effect. She taunted Harry that he needed to really "enjoy" giving pain in order for it to work, as he learned later when he used Cruciatus on Amycus Carrow during the Battle of Hogwarts (DH30).
• Snape killed Dumbledore using the Killing Curse on top of the Astronomy Tower (HBP27).
• Bellatrix tortured Hermione with the Cruciatus Curse at Malfoy Manor (DH23).