MagicPhenomenon
Priori Incantatem
Priori Incantatem is the reverse-spell effect that happens when two wands sharing a core are forced to duel. The wand that loses regurgitates echoes of its last spells. In the graveyard, Voldemort’s wand, twin to Harry’s, releases shades of Cedric, Frank Bryce, Bertha Jorkins, and James and Lily Potter.

First mention
Harry Potter and the Goblet of FireChapter: Priori Incantatem
Harry and Voldemort’s spells lock, a web of gold light cages the duel, and the echoes step out of Voldemort’s wand.