Priori Incantatem

Prior Incantato, otherwise known as the Prior-Use Spell, reveals the magic most recently performed by a wand. It was used by Amos Diggory to find out whether Harry’s wand had been used to cast the Dark Mark at the Quidditch World Cup. There is also Priori Incantatum, the reverse spell effect. This occurs when wands with twin cores meet and are unable to perform magic properly against one another. One wand will force the other to regurgitate the spells it has performed in reverse. It happened with Harry and Voldemort in the graveyard in Goblet of Fire, resulting in Harry seeing the echoes of Voldemort’s victims.