Reverse Spell

Priori Incantatem is the effect which occurs when two wands which share the same source for their cores compete in combat.

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.

The only known instance of Priori Incantatem came in 1995, in a graveyard in Little Hangleton, when Harry Potter first dueled with the resurrected Lord Voldemort.