Priori Incantatem

"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. 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 is a magical phenomenon that can occur when two wands which share the same source for their cores are forced to compete in combat. It is sometimes referred to as the "reverse-spell effect". It can also be induced by analytical spells, like an extension of the Prior Incantato spell.

Effects
During a wand duel, a simultaneous spellcasting by both parties with twin wand cores will trigger an effect whereby both wands are linked through separate threads of spell energy. 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. When the spells come into contact they send out a seismic pulse which has enough force to knock onlookers from their feet. When the spells meet, they intertwine, making it difficult for either caster to break the connection. A spell-like residue falls from the core of the connection, possibly representing the wills of the casters and the energy of the incantations. A dome effect occurs during the connection, in order to keep any intruders from attacking either wizard or witch. The sound of the connection is similar to electrical arcs.

Similarities
The effect is similar to the spell Prior Incantato (but this spell does not need a wand connection), which causes the targeted wand to recall its last spells.

Known instances
The only widely-known instance of Priori Incantatem came about in 1995, at the graveyard in Little Hangleton, 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. Due to rarity of twin cores ever meeting up with each other, this phenomenon is also rare.


 * Priori Incantatem rarely ever happens when two opponents forcibly use their wands in battle. The reason for the case of the 1995 duel between Harry Potter and Lord Voldemort are the shared wand cores; since the wands recognised each other as "brothers", they refused to destroy each other. When spurred on, however, the dominant (Harry's) forced the submissive to regurgitate its spells.

Etymology
A possible etymology for Priori Incantatem is prior incarnation, refering 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.

Behind the scenes.

 * During the duel, Voldemort's wand emitted "echoing screams of pain", which may refer to the Cruciatus Curse he used earlier on both Harry and Wormtail (when punishing the latter for letting Barty Crouch Sr. escape).
 * Throughout the film adaptations there are several instances in which two wizards, while duelling, find themselves locking spells with their opponent, using Priori Incantatem. However, this may be disregarded, as all occurrences (save for the one in Goblet of Fire) lack key elements of Priori Incantatem, such as the "regurgitation" of spells performed by the wands, the "golden cage" around the casters, and twin wand cores. David Yates has admitted that he did want wands to connect in the movies for 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.

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