Reverse Spell

Priori Incantatem is called "the reverse-spell effect." It can occur when two wands which share the same source for their cores are forced to compete in combat. 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 will trigger an effect where both wands are linked through a golden thread of 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.

The effect is similar to the spell Prior Incantato, which causes the targeted wand to recall its last spells.

Known Instances
The only widely-known instance of Priori Incantatem came in 1995, at the graveyard in Little Hangleton, when Harry Potter first dueled with the resurrected Lord Voldemort. The two wands in question both had a phoenix feather at their core from Fawkes, pet of Albus Dumbledore.


 * 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.

Behind the scenes

 * Voldemort's victims were supposed to come out in reverse order. However, since James Potter was killed before Lily, he should have come second after Lily. 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.
 * 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).