False memory charm

"Then he returned to the Gaunt hovel, performed the complex bit of magic that would implant a false memory in his uncle's mind, laid Morfin's wand beside its unconscious owner, pocketed the ancient ring he wore, and departed."

- Albus Dumbledore theorizes, correctly, that Voldemort used this spell. The false memory charm is a spell which can be used by a witch or wizard in order to add a memory to their victim's mind without the victim realising it wasn't originally theirs. It differs from the Memory Charm in that the latter erases memories whilst this one adds them.

Known uses

 * Voldemort used this spell twice; firstly in 1943 upon his uncle Morfin Gaunt, then sometime later on the House-elf Hokey. Both instances were to hide his crimes and make the victims believe they were responsible.
 * It is likely that Kingsley Shacklebolt used this spell on Marietta Edgecombe in 1996 in order to prevent her from revealing Dumbledore's Army to Cornelius Fudge. This is reinforced by Dumbledore stating that Shacklebolt was "remarkably quick on the uptake" in modifying her memory.
 * Later that same year, Voldemort used the spell to make Harry believe his godfather Sirius Black was being tortured by Voldemort in the Department of Mysteries. However, it is possible that Voldemort simply created an image in Harry's mind through his tight Occlumens connection with Harry (due to the Horcrux connection) so that Harry would see what Voldemort's imagination dreamt up. It may have been up to Harry whether or not to believe it since he is persuaded by Hermione to check to see whether Sirius is really in the Department of Mysteries (not in Grimmauld Place). Voldemort is relying, therefore, on Harry's concern for Sirius's safety to get him to come, just as he is sure Harry will come to him in the forest during the break in the Battle of Hogwarts.
 * It is possible that Professor Horace Slughorn used this in the next school year to hide his memory of Voldemort asking him about Horcruxes, although as he retained awareness of the true memory his spell may be a different one.
 * Hermione Granger used this in 1997 to make her parents believe they were named Wendell and Monica Wilkins respectively to make them move to Australia in order to protect them from Voldemort and his Death Eaters.

Behind the scenes

 * The actual name and incantation for this charm are unknown.
 * However, it appears to be reversible as Hermione stated that she would find her parents after the war and lift it.
 * Albus Dumbledore could reverse this spell, as he managed to uncover the true memories beneath this one (he admitted that it was rather difficult).
 * In the film, Hermione uses Obliviate instead of this spell.

Appearances

 * Harry Potter and the Order of the Phoenix
 * Harry Potter and the Order of the Phoenix (film)
 * Harry Potter and the Half-Blood Prince
 * Harry Potter and the Half-Blood Prince (film)
 * Harry Potter and the Deathly Hallows