Reverse Spell

Priori Incantatem (a.k.a. the reverse-spell effect) is an effect where the previous spell(s) cast by a wand are revealed. It can occur when two wands wands sharing the same source for their cores are forced to compete in combat. It can also be magically induced, e.g. by the charm Prior Incantato.

Effects
During a wand duel using wands whose cores are the same material from the same source, a simultaneous spell-casting by both parties will trigger an effect where both wands are linked through a golden thread of energy. This energy also seems to create a dome of some sort around the two duellists that others are unable to penetrate. In the case of wands with Phoenix feather cores, a sound like Phoenix song is heard. The two wand holders then compete in a battle of wills, in which the loser's wand is forced to display in ghostly form the spells which had been cast by said wand, in order of most recent to least recent. If any of those spells are Killing Curses, the victims will appear in ghost-like form, capable of holding a conversation with nearby living beings. It also makes the two casters hover for a short period of time.

The effect is similar to the spell Prior Incantato, which causes the targeted wand to reveal the spells it previously cast in reverse order.

Known Instances

 * The only widely-known instance of Priori Incantatem came at the Little Hangleton graveyard, when Harry Potter first duelled with the resurrected Lord Voldemort. The two wands in question both had a phoenix feather at their core from Fawkes, pet of Albus Dumbledore. Voldemort's will lost, and he was forced to suffer the echoes of the people he murdered with the Killing Curse, all of whom were also supporting Harry.
 * Priori Incantatem rarely ever happens when two opponents forcibly use their wands in battle. The reason it happened during the 1995 duel between Harry Potter and Lord Voldemort is due to the cores their wands shared; since the wands recognized each other as "brothers," they refused to destroy each other, and when spurred on, one forced the other to regurgitate its spells.
 * In his final duel with Voldemort during the Battle of Hogwarts, Harry again used a Disarming Charm against Voldemort's Killing Curse, repeating the event of 1995 with the exception of a Priori Incantatem effect taking place. This time, it was Harry's rightful ownership of the Elder Wand, being wrongly used by Voldemort, that led to Harry's victory.

Behind the scenes

 * Voldemort's victims were supposed to come out in reverse order. Therefore, since James Potter was killed before Lily, he should have come second after Lily. The original publication of the book identified James Potter being first to show up on the scene. This was a minor mistake in the series, which was corrected in later editions.
 * As Bertha Jorkins was omitted in the film adaptation, she did not appear in the echo.
 * During the duel, Voldemort's wand emitted "echoing screams of pain," which may refer to the Cruciatus Curse he used earlier that evening on both Harry and Avery, as well as his torture of Wormtail for allowing Barty Crouch Sr. to escape.
 * In the film adaptation of Harry Potter and the Order of the Phoenix, Albus Dumbledore and Lord Voldemort's wand connect as if they were giving another example of Priori Incantatem, but there is no explanation from canon as we know that could explain this event. However, the director had admitted that he did want their wands to connect in the movie for effect. It was most likely not Priori Incantatem because no images of prior spells appeared. The most likely explanation is that since Dumbledore and Voldemort are both great wizards, their spells would just block the others, which explains the seemingly random light tendrils they create.
 * A trailer for Harry Potter and the Deathly Hallows: Part 2 depicts Harry in his final duel with Voldemort and the same green and red tendrils appearing.

Appearances

 * Harry Potter and the Goblet of Fire
 * Harry Potter and the Goblet of Fire (film)
 * Harry Potter and the Goblet of Fire (video game)
 * Harry Potter and the Order of the Phoenix (film)
 * Harry Potter and the Deathly Hallows
 * Harry Potter and the Deathly Hallows: Part 1
 * Harry Potter and the Deathly Hallows: Part 2
 * Lego Harry Potter: Years 1-4

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