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Dark Arts
The Dark Arts are magical spells and practices that are usually used for malicious purposes. Practitioners of Dark Arts are referred to as Dark witches or wizards. The most prominent of these is Voldemort, known to them as the Dark Lord. His followers, known as Death Eaters, practice the Dark Arts while doing his bidding.
The type of spells characteristic of Dark Arts are known as curses, which usually cause harm to the target. All, to a certain degree, are in some circumstances justifiable. The motivation of the caster affects a curse's result. This is most clearly outlined for the Cruciatus curse: When cast by Harry, angered by the death of his godfather at Bellatrix's hands and desiring to punish her, it causes a short moment of pain. As Bellatrix herself comments, righteous anger does not allow the spell to work for long. When cast by figures such as Voldemort, who desire to inflict pain for its own sake, it causes intense agony that can last as long as the Dark witch or wizard persists.
Use of Dark Magic can corrupt the soul and body; Voldemort has used such magic in his quest to prolong his life and obtain great power. The Dark Arts also cause Voldemort to look deformed and inhuman, a side effect of splitting his soul into Horcruxes.
According to Snape, the Dark Arts "are many, varied, ever-changing and eternal ... unfixed, mutating, indestructible".[HBP Ch.9] In magical dueling, there are any number of spells that may be used to attack, immobilise, or disarm an opponent without causing pain or lasting harm; however, spells such as the Cruciatus Curse wound or seriously distress a victim in some way. Dark spells can be classified into three groups: Jinxes, hexes, and curses.
In the wizarding world, use of the Dark Arts is strongly stigmatised and extreme forms are illegal; however, these spells are prevalent enough that even before the rise of Voldemort, many schools (including Hogwarts) taught Defence Against the Dark Arts as a standard subject. Techniques include anti-curses and simple spells to disable or disarm attackers, or fight off certain creatures. Some schools, such as Durmstrang, teach Dark Magic. A Dark Arts class is also taught at Hogwarts while it is under Death Eater control.Unforgivable Curses
The Unforgivable Curses are the most powerful known Dark Arts spells. They were first classified as unforgivable in 1717. Used by the books' villains, such as Voldemort and the Death Eaters and in some cases the Ministry of Magic, their use inspires horror and great fear amongst others. The curses are so named because their use is forbidden and unforgivable in the wizarding world and is punishable by a life sentence in Azkaban. The only exception is if a person is proven to have done them under the influence of mind control. These curses are thus very rarely used openly.The Killing Curse (incantation: Avada Kedavra) manifests as a jet of green light that causes immediate death. The Killing Curse has no counter-curse and cannot be blocked by most magical means, although it can be blocked by love (e.g. Lily Potter giving up her life for her son). However, the green energy bolt may be dodged, blocked by solid objects, or intercepted with a few other powerful fast spells, particularly Stunning spells.
The Cruciatus Curse (incantation: Crucio, Latin for "I torture") causes the victim intense pain and is used for torture, but does not physically harm them. The strength of the curse is determined by the person who cast it. It was used regularly by the Death Eaters. The curse can  torture a person to the point that they are exhausted to death, or in a more sadistic case, it causes permanent amnesia and insanity. Effective casting requires the caster to have sadistic desires. It is shown as a bright-faded blue light (though it is usually invisible or even a red bolt of light in the films).
The Imperius Curse (incantation: Imperio, dog Latin for "I command") is used for mind control or hypnosis and can force the victim to do things they would normally be unwilling or unable to. The strength and duration of the curse depends on the caster, as well as the level of resistance of the victim. The curse is shown as a bright haze (while it was either invisible or green, mist-like haze in the films).The use of the Unforgivable Curses was authorised against Voldemort and his followers by Bartemius Crouch Sr, during the First Wizarding War. (Shortly after his resurrection, Voldemort names two Death Eaters "killed by Aurors".) In addition, in Deathly Hallows, the Unforgivable Curses are used liberally by good characters, ranging from Professor McGonagall with the Imperius Curse, to Harry effectively using the Cruciatus Curse. He also uses the Imperius Curse on a goblin and a suspicious Death Eater during their disguised attack upon Gringotts Bank.
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