Patronus Charm

The Patronus Charm, expecto patronum, conjures a silvery phantom shape, usually that of an animal, which is the embodiment of the positive thoughts of the caster. A Patronus will drive away Dementors.

A fully-shaped spirit animal is called a “Corporeal Patronus.” (OP8, OP16)

References from the canon

• The Patronus Charm is one of most ancient types of magic, yet there has never been a way to predict what Patronus a witch or wizard will have (Pm)
• Lupin taught Harry to cast this spell, which he performed with minor success until he faced a large group of Dementors who were trying to attack Sirius Black. Harry saw a Patronus come charging across the lake and later realized that he himself cast it (PA12, GF31).
• Harry used his Patronus Charm to drive off two Dementors in an alley near Privet Drive. He got into trouble for doing it, although he had no honourable alternative under the circumstances (OP1).
• Madam Bones at the Ministry was impressed that Harry could create a corporeal Patronus at his age (OP8). Her niece Susan Bones asked Harry about it during the first meeting of Dumbledore's Army at the Hogshead (OP16).
• Harry cast a fully-formed Stag Patronus to earn an extra point on his O.W.L. Exam. (OP31)
• Cast by Harry, then by Hermione at the Ministry of Magic, on which occasion Harry said that it is the only spell that Hermione has trouble with (DH13).
• A Dark wizard named Raczidian tried to cast a Patronus Charm and was instead eaten by maggots that emerged from his wand (Bloomsbury New Editions, 2015)
• Rubeus Hagrid was unable to produce a proper patronus because the spell was too difficult for him (JKR:tw).