Reverse Spell

"A jet of green light issued from Voldemorts 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... he felt his feet lift from the ground. He and Voldemort were both being raised into the air, their wands still connected by that thread of shimmering golden light."

—The Harry's and Voldemort's twin wand cores connect^[src]

The Reverse Spell^[2] (Prior Incantato)^[1] was a spell that forced a wand to show an "echo" of the most recent spell it had performed.^[1]^[3]^[4] An especially rare manifestation of this spell is known as Priori Incantatem.^[3]^[4] It is an unintended consequence of two wands with cores taken from the same animal, duelling each other; the winner's wand forces the loser's wand to reproduce in reverse order "echoes" of the most recent spells performed.^[3]^[4]

Effects

Priori Incantatem

"The golden thread connecting Harry and Voldemort splintered; though the wands remained connected, a thousand more beams arced high over Harry and Voldemort, crisscrossing all around them, until they were enclosed in a golden, dome-shaped web, a cage of light..."

—The dome of light, a side-effect of the spell^[src]

Wormtail graveyard — The cage of protective light created by 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.^[3]^[4] Both wands became linked through a single, golden thread of energy.^[3] Hundreds of smaller golden beams shot off from this central thread to form a cage around the duellers.^[3]

The two wand holders were then forced to compete in a battle of wills.^[3] 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.^[3]^[4]

HP4GoF4433 — The "echo" of Cedric Diggory released by the effects of Priori Incantatem

If one of these spells was the Killing Curse, then a ghostly "echo" of the murdered person could emerge from the killer's wand.^[3]^[4] Such "echoes" retained the "appearance and character" of the deceased person.^[3]^[4] They were able to hold conversations and could remember the events leading up to and following their deaths.^[3]^[4] However, these "echoes" were not ghosts, nor were they considered to be returning the deceased person from the dead^[4]; indeed, they could only linger on the mortal plane for a few moments before they faded back into the Beyond.

History

Duel little hangleton — Harry Potter and Lord Voldemort duel in Little Hangleton graveyard

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.^[1]

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.^[3]^[4] Voldemort's wand was ultimately overcome by Harry's, causing it to release "shadows" of people he had murdered with it: Cedric Diggory, Frank Bryce, Bertha Jorkins, as well as Lily and James Potter.^[3]^[4]

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. Although similar to Priori Incantatem, these events did not actually result in a cage of light, nor the release of echoes of past spells.