Notes

Defense Against Dark Arts
@MizukiHimeji_XO
Ravenclaw


Expecto Patronum, or the Patronus Charm, will cast a Patronus, which can appear as simply white vapour, or in more advanced casters, as a silvery-white animal shape. If it takes the shape of an animal, it is called a corporeal Patronus. This spell is used to ward off the Dementors, which are the guardians of Azkaban.

=> History
It is evident from old woodcuts and scrolls that the Patronus Charm has been used since ancient times. Therefore, it is unknown who created it, or when it was first invented. The charm also has a long association with those fighting for lofty or noble causes (those able to produce corporeal Patronuses were often elected to high office within the Wizengamot and Ministry of Magic).

Harry Potter took anti-dementor lessons in order to learn the advanced charm and so was one of the youngest to attempt this charm with actual results. During a Quidditch match against Ravenclaw in 1994 he cast his very first corporal Patronus and again in June when he along with Hermione Granger and Sirius Black were being attacked by over a hundred Dementors, he cast a corporal Patronus powerful enough to drive them all away.

In 1995, Harry cast the charm against a boggart that he thought was a dementor during the third task of the Triwizard Tournament. The following summer he came under attack by two Dementors and managed to drive them away with the charm, leading to him being put on trial for underage magic in an attempt to discredit him.

Harry also taught the members of Dumbledore's Army the charm and some were even successful in casting it.

The charm was used frequently to ward off dementors such as when the trio escaped from the Ministry of Magic after obtaining the Horcrux locket and during the Battle of Hogwarts. Severus Snape also conjured a Patronus to guide Harry to the hiding place of the Sword of Gryffindor.

The Order of the Phoenix would send out messages with their Patronuses such as Kingsley Shacklebolt and Arthur Weasley who in 1997 delivered warnings.

=> Difficulty
The Patronus Charm is widely regarded as advanced magic, far beyond N.E.W.T.-level; in 1994 Remus Lupin stated that the charm was, in fact "ridiculously advanced". This charm was, indeed, so perilous, few wizards/witchs, could conjour up a true Patronus. It is very complex and many qualified wizards and witches have trouble with it. In fact, Harry Potter is one of the youngest known wizards able to cast a Patronus; he was taught how to do so in early 1994 at the age of thirteen by Remus Lupin. In a Dumbledore's Army lesson, Harry taught the members how to use the charm. Some were even successful in casting a corporeal form, though Harry said this might be because there was no Dementor to make them frightened. However, three members were later able to cast corporeal Patronuses in the presence of Dementors.[10] Being able to cast a patronus, whilst fighting Dementors or not, is not required to graduate from Hogwarts so it is something that is taught under special circumstances.

Amelia Bones and potential members of the D.A. were all very impressed when Harry Potter revealed that he could cast not only a Patronus Charm but a corporeal one to boot, which is notably more difficult than casting an incorporeal one. Given that the success of the charm is directly reliant on the caster retaining a particular mental state the Charm is more difficult to cast in emotionally trying circumstances. For instance, Harry Potter, Ron Weasley, and Hermione Granger all had difficulty casting their Patronuses when under the negative influence of Salazar Slytherin's Locket, one of Lord Voldemort's Horcruxes, and after witnessing the death of Fred Weasley in the final battle. This feature of the Charm is particularly unfortunate since the Charm's primary use is to defend against Dementors, Dark creatures specifically equipped to mentally unhinge people (and hence is partly why it is regarded as such advanced, difficult magic).

Self-confidence may also play an important role in casting the spell. In 1994 Harry is able to cast the spell successfully, driving off a large number of Dementors (which according to Severus Snape is only achievable by a very powerful wizard) thus saving Sirius and a younger version of himself, seemingly without recalling a happy memory, when he realises that he has already done it. Although Harry could have had a 'happy memory' upon realising that he had saved all three of their souls.

Worthiness

It is a general belief held by the wizarding world that only those who are pure of heart are able to cast Patronuses; this, however, is untrue, as several characters with negative personality traits are able to cast a fully-fledged Patronus. Although generally Dark witches and wizards will not try to produce a Patronus, not having any need for one, most Dark wizards will be devoured by maggots coming from their wand and consuming the caster.

For this reason, Lord Voldemort and his Death Eaters were not able to cast a Patronus Charm (not that it was necessary, as they already had the dark creatures under their control via their affinity to darkness, and would have no use for Patronuses). However, some witches and wizards of questionable morals, such as Dolores Umbridge, are able to produce corporeal Patronuses, Umbridge doing so despite the fact that she was considered an evil person; her wearing of Salazar Slytherin's Locket, which was one of Voldemort's Horcruxes, was what enhanced her strength due to her evil affinity. She was able to produce a corporeal Patronus due to the enhanced strength. Conversely, Draco Malfoy was unable to cast one despite his waning evil. Severus Snape is the only Death Eater capable of casting the charm, as his love for Lily Evans was his one redeeming point.