Untransfiguration

"Merlin's beard! No need to disfigure me, Albus."

- Horace Slughorn to Albus Dumbledore

Untransfiguration is a type of counter-spell that is the exact opposite of Transfiguration i.e. it returns something to its proper form, reversing the effects of a transfiguration.

Reparifarge seems to be a general Untransfigurative spell used to counter the effects of a Transformation spell that was poorly performed and only partially transfigured the object in question. Probably, there are other Untransfigurative spells, and these are possibly taught at N.E.W.T.-level.

1991–1992 school year
In Harry Potter's first Transfiguration class, in 1991, when Professor Minerva McGonagall Transfigured her desk into a pig, and the pig back to a desk.

1993–1994 school year
Also in 1993 when Remus Lupin and Sirius Black changed Peter Pettigrew from a rat back into his usual human appearance.

Known practitioners

 * Minerva McGonagall
 * Remus Lupin
 * Sirius Black
 * Aurors

Behind the scenes

 * According to W.O.M.B.A.T., it may not be possible to Untransfigure a person or object without knowing what they were originally. However, in Harry Potter and the Goblet of Fire, when Draco Malfoy is turned into a ferret, Professor McGonagall Untransfigures him while only knowing that it was a student, not who it was specifically. This implies that either one does not need to know very specific details about what the object was before Transfiguration or that she guessed it was Malfoy due to his absence from his usual gang.

Appearances

 * Harry Potter and the Philosopher's Stone
 * Harry Potter and the Chamber of Secrets (film)
 * Harry Potter and the Prisoner of Azkaban
 * Harry Potter and the Prisoner of Azkaban (film)
 * Harry Potter and the Goblet of Fire
 * Harry Potter and the Goblet of Fire (film)
 * Harry Potter and the Order of the Phoenix
 * Pottermore

Notes and references
Destransformación