Reverse Spell

The Reverse Spell (Priori Incantato ) was a spell that forced a wand to show an "echo" of the most recent spell it had performed.

An especially rare manifestation of this spell was known as Priori Incantatem. It occurred as an unintended consequence of two wands with identical cores having to duel each other, and resulted in the winner's wand forcing the loser's wand to reproduce "echoes" of the most recent spells it had performed in reverse order.

Priori Incantatem
When a duel occurred between wizards with wand cores harvested from the same creature, simultaneous spell-casting by both parties would trigger a rare, powerful effect called Priori Incantatem. Both wands became linked through a single, golden thread of energy. Hundreds of smaller golden beams shot off from this central thread to form a cage around the duellers.

The two wand holders were then forced to compete in a battle of wills. When one wand managed to overcome the other, it forced the losing wand to regurgitate "echoes" of the most recent spells it had cast, in reverse order.

If one of these spells was the Killing Curse, then a ghostly "echo" of the murdered person could emerge from the killer's wand. Such "echoes" retained the "appearance and character" of the deceased person. They were able to hold conversations and could remember the events leading up to and following their deaths. However, these "echoes" were not ghosts, nor were they considered to be returning the deceased person from the dead.

1994-1995
Amos Diggory cast the Reverse Spell on Harry Potter's wand in the aftermath of the Death Eater attack at the 1994 Quidditch World Cup in order to determine whether he conjured the Dark Mark over the campsite.

Lord Voldemort was resurrected by Peter Pettigrew using the blood of Harry Potter in the Little Hangleton graveyard on 24 June 1995. A duel ensued between Voldemort and Harry, during which their wands (which both contained feathers from the phoenix Fawkes) became connected, triggering Priori Incantatem. Voldemort's wand was ultimately overcome by Harry's, causing it to release "shadows" of people he had murdered with it: Frank Bryce, Bertha Jorkins, Cedric Diggory, and James and Lily Potter.

Behind the scenes

 * Throughout the film adaptations of the story, there are several instances of two wizards locking spells with one another, presumably for dramatic effect. Instances of this include Albus Dumbledore and Lord Voldemort during the duel in the Ministry Atrium in Harry Potter and the Order of the Phoenix (film); a Death Eater and Arthur Weasley in the Battle of Hogwarts in Harry Potter and the Deathly Hallows: Part 2; and Harry Potter (using Draco Malfoy's wand) and Lord Voldemort (using the Elder Wand) several times during Voldemort's Last Stand, also in
 * In Harry Potter and the Deathly Hallows: Part 1, during the Skirmish at Malfoy Manor in 1998 Bellatrix Lestrange used this spell on Hermione Granger's wand to learn if she used a Stinging Hex on Harry Potter to make him unrecognisable.


 * In the novel of Harry Potter and the Goblet of Fire, Bertha Jorkins' echo appears during Voldemort's duel with Harry, while in the film adaptation, her echo doesn't appear. This is because the character of Bertha Jorkins has been omitted entirely from the films.