Avada kedavra instance of magic spell in Harry Potter
Priori Incantatem
Priori incantatem (from Latin: priorem incantatum – a previous spell), or the reverse spell effect, is used to display the spells cast by a wand. The spells cast by the wand will emerge in smoky or ghost-like replicas in reverse order, with the latest spell emerging first. It is first encountered in Goblet of Fire when Barty Crouch Sr.'s house elf Winky is caught holding Harry's wand. This spell is used to reveal that it was indeed Harry's wand that was used by Crouch's son to cast the Dark Mark. In Harry Potter and the Half-Blood Prince, it is revealed that the teenage Voldemort murdered his father and grandparents using his uncle Morfin's wand, knowing that, when examined, the wand would incriminate Morfin as the murderer. In Deathly Hallows, Harry fears that a priori incantatem spell will be used on Hermione's wand after the Death Eaters get hold of it. This would reveal that she had accidentally broken his holly-and-phoenix-feather wand when a curse misfired and they both narrowly escaped Voldemort earlier. As a result, the protection of the shared cores was lost and, worse still, this would now be made known to Voldemort. During the final duel between Harry and Voldemort, the latter mentions that he knows that the holly and phoenix wand is destroyed, implying that the priori incantatem has indeed been performed on Hermione's wand, as they had feared.
Forcing two wands that share the source of their cores to do battle can also cause a more potent form of priori incantatem. The tips of the two wands will connect, forming a thick golden "thread" of energy, and the two wands' masters fight a battle of wills. The loser's wand will regurgitate shadows of spells that it has cast in reverse order. This phenomenon occurs during the duel between Harry and Voldemort at the end of Goblet of Fire. Their simultaneous spells (Harry casts "expelliarmus" and Voldemort casts "avada kedavra") trigger the threads, and as Voldemort loses the battle of wills, his wand regurgitates, in reverse order, echoes of the people his wand had most recently murdered: Cedric Diggory; Frank Bryce; Bertha Jorkins; as well as Harry's parents.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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