Expecto Patronum is the incantation for the Patronus Charm in the Harry Potter books and films, most famously cast by Harry Potter himself. Taught by Remus Lupin in Harry Potter and the Prisoner of Azkaban, the spell requires a powerful happy memory and produces a guardian made of positive energy. Harry's Patronus takes the form of a stag, like his father's. The line became one of the series' most quoted spells, and short clips of the shout spread on soundboards and in edits as a shorthand for protection and hope.