Unforgivable Curses

The Unforgivable Curses, as observed in J.K. Rowling's Harry Potter series, are the most powerful Dark Arts spells known. They are named so because they are forbidden and unforgivable to use without Ministry of Magic permission. Getting caught using one results in the perpetrator spending the rest of their life in Azkaban.

As the spells are very strong, their use requires a strong desire to bring about the effects, a directed will, and no small amount of skill.

The Curses are most often used by villains such as Voldemort and the Death Eaters. However, in the final few books of the series, many of the protagonists use the Curses openly and without consequence against antagonists.

Avada Kedavra (The Killing Curse)
Pronounced "uh-VAH-duh kuh-DAHV-ruh". It causes a flash of bright green light and a rushing sound, then brings instant death to the victim. There is no known counter-curse or blocking spell, though the victim may dodge, hide behind solid objects (which burst into flame after being hit), or, if the caster is not competent, the curse may be ineffective.

The only known survivor of this curse is Harry Potter.

Crucio (The Cruciatus Curse)
Pronounced "KROO-see-oh". It causes a flash of red light and brings unbearable pain to the victim. The effects of the curse depend on the desires and emotions of the caster. According to Bellatrix Lestrange, to achieve the excruciating pain implied by its name, the caster must desire to cause pain purely for its own sake. The extreme pain inflicted by the curse without leaving any physical marks upon the body makes it a perfect form of torture, and is used regularly by the Death Eaters.

Imperio (The Imperius Curse)
Pronounced "im-PEER-ee-oh". This curse places the victim in a trance or dream-like state and leaves them subject to total control to the spoken and unspoken commands of the caster. The experience of being controlled by the curse is described as a complete release of responsibility or worry over one's actions at the price of free will. Resisting the curse, however, is possible, and several individuals have learned to successfully and easily throw it off after being subjected to its effects for an extended period of time.