Polyjuice is a potion that allows to take someone else's physical appearance.
Counter-spell[]
The incantation polyfluis reverso can reverse the effects of polyjuice.
Known uses[]
On the day when Harry and Professor Quirrell break Bellatrix Black out of Azkaban, Dumbledore, Snape, and McGonagall fear for Harry's safety so Snape uses polyjuice to take Harry's place and act as bait.