<P> 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 and Voldemort cast "Expelliarmus" and "Avada Kedavra" respectively) trigger the threads, and as Voldemort loses the battle of wills, his wand regurgitates, in reverse order, echoes of the people his wand had last murdered: Cedric Diggory; Frank Bryce; Bertha Jorkins; as well as Harry's parents . Harry was previously informed by Mr. Ollivander that the holly wand that "chose" Harry was the "brother" of the yew wand that gave him the lightning - shaped scar on his forehead, although the significance of this was not discussed then . Dumbledore later reveals to Harry that his and Voldemort's wands both contain a tail feather given by Dumbledore's pet phoenix, Fawkes . </P> <P> 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 . </P> <P> 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 notable in the case of Cruciatus: 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 desires . 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 . </P> <P> According to Snape, the Dark Arts "are many, varied, ever - changing and eternal...unfixed, mutating, indestructible". 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 or Sectumsempra, judged to be Dark by reliable authorities, wound or seriously distress a victim in some way . Dark spells can be classified into three groups: jinxes, hexes & curses . </P>

When did harry potter use the cruciatus curse
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