Reverse Spell

"A jet of green light issued from Voldemort's wand just as a jet of red light blasted from Harry's — they met in midair — and suddenly Harry's wand was vibrating as though an electric charge were surging through it; his hand seized up around it; he couldn't have released it if he'd wanted to — and a narrow beam of light connected the two wands, neither red nor green, but bright, deep gold. Harry, following the beam with his astonished gaze, saw that Voldemort's long white fingers too were gripping a wand that was shaking and vibrating."

- Description

Priori Incantatem, sometimes referred to as the reverse-spell effect, was a magical phenomenon that occurred when two wands sharing the same cores were forced to compete in combat.

The effect could be induced using the incantation Prior Incantato. The echos caused by this spell, if conjured by incantation, may be destroyed by means of the Eradication Spell.

1994
In the woods at the 1994 Quidditch World Cup in Dartmoor, England, this effect was invoked by Amos Diggory by incantation on Harry Potter's wand, which had been wielded by Winky the house-elf, in order to determine who had conjured the Dark Mark over the campsite.

1995


In the Little Hangleton Graveyard on 24 June, 1995, the Dark Lord Voldemort was resurrected from the dead with the blood of Harry Potter. A duel ensued, in which the wands, both of which possessed phoenix feathers from Fawkes, pet of Albus Dumbledore, were locked together in golden light, and Voldemort's wand, submissive to Harry's, created shadows of Frank Bryce, Bertha Jorkins, Cedric Diggory and James and Lily Potter.

Effects


Where a duel occurs between parties with wand cores from the same source, simultaneous spell-casting by both witches or wizards will trigger an effect whereby both wands are linked through a single, golden thread of energy, off of which shoot hundreds of beams of golden light forming a cage around the casters.

The two wand holders are forced to compete in a battle of wills in which the loser's wand is forced to regurgitate ghostly echoes of previous spells cast by said wand in reverse order of which they were cast. The victims of murder are able to hold conversations with those around them, and are capable of remembering the events leading up to and even after their deaths.

Etymology
A possible etymology for Priori Incantatem is prior incarnation, referring to its effect of "regurgitating" previous spells. However, the Latin literally means "prior incantation" or "previous spell"; more literally, an incantation is a spell spoken aloud.

Etymology
Latin prior, "previous", and incantare, "to speak a spell" (past participle incantatum).

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; 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 Harry Potter and the Deathly Hallows: Part 2.
 * Peter Pettigrew took Voldemort's wand after the attack on Godric's Hollow failed, due to fearing that the Ministry would use this spell to check the wand to discover Pettigrew's treason. This suggests that the wand has record of Pettigrew's affiliation (such as branding the Dark Mark onto his arm.
 * 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 unrecognizable.

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 Deathly Hallows: Part 1 
 * Harry Potter: A Pop-Up Book
 * LEGO Harry Potter: Years 1-4