The Cruciatus Curse (also known as the Torture Curse) (Crucio) is a tool of the Dark Arts and one of the three Unforgivable Curses. It is one of the most powerful and sinister spells known to wizardkind. When cast successfully the curse inflicts intense, excruciating pain on the victim. The use of this curse on another human being is a life-long sentence in Azkaban.
Considering that the curse does not physically harm the victim, it is possible that it only stimulates pain receptors. The two most well-known victims of this curse are the former Aurors Alice and Frank Longbottom. Frank and Alice were tortured into insanity with it by Death Eaters: Bellatrix, Rodolphus, Rabastan Lestrange, and Barty Crouch Jr.