Reverse Spell effect

The Reverse Spell Effect, known as Priori Incantatum, forces images of past spells out of a wand (GF9).

References from the canon

• Can be cast as a spell, ("Prior Incantato"), in which case it forces the target wand to emit a ghost image of the last spell it cast. The images can be dispelled using Deletrius . Amos Diggory was able to prove that Harry's wand had performed Morsmordre to put the Dark Mark in the sky over the Quidditch World Cup Campground (GF9).
• When two wands are forced to duel that have core material from the same single creature, the result will be "Priori Incantatem," a display in sequence of the last spells one of the wands cast. Which wand will show the spell effect depends on the willpower of the two wizards involved (GF34, GF36)
• Ghost figures of some of those murdered by Voldemort's wand emerged from Harry's wand in reverse order to surround him protectively in the "golden web" created by the Priori Incantatem: Cedric Diggory, Frank Bryce, Bertha Jorkins, Lily Potter, and lastly, James Potter (GF34).
• Mr. Ollivander told Voldemort that he could avoid Priori Incantatem by using a different wand without a phoenix-feather core, so he took Lucius Malfoy's elm wand. However, because Harry's holly wand absorbed some essence of Voldemort's power during Priori Incantatem, it blew up the elm wand during the Battle of the Seven Potters (DH4, DH5, DH24,DH35)