Boggart

"So the boggart sitting in the darkness within has not yet assumed a form. He does not yet know what will frighten the person on the other side of the door. Nobody knows what a boggart looks like when he is alone, but when I let him out, he will immediately become whatever each of us most fears."

- Professor Lupin to his Third year class in 1993

A boggart is an amortal shape-shifting non-being that takes on the form of the viewer's worst fear. When facing a boggart, it is best to have someone else along, to try to confuse it, since facing more than one person at once would make it indecisive as to what form it must take, usually a mixed-up amalgam of the victims' fears. Because of their shape-shifting ability, no one knows what a boggart looks like when it is alone, as it instantly changes into your worst fears when you first see it.

The charm that combats a boggart is Riddikulus. The charm requires a strong mind and good concentration. The incantation and wand movement alone will not affect a boggart. The correct way to perform the charm is to push past the fear, and concentrate on something that will make the boggart look amusing. The charm does not, in fact, repel a boggart; it just forces it to assume a shape that the caster will find comical, inspiring laughter, which will defeat a boggart.

Boggarts sometimes do not have the same strength or magic as what they imitate, for example, a Boggart transformed into a Dementor has weaker magic than a real Dementor, according to Lupin, or having a deadly scream as a true Banshee, which would otherwise have slaughtered the entire class. This was partly why he chose to simulate a Dementor attack in Harry's presence to help the boy perfect the use of the Patronus Charm.

Muggles can sometimes see Boggarts, but rarely see them plainly and are often easily convinced that they are a figment of their imagination.

Hiding places
Boggarts are particularly fond of inhabiting certain places that are dark, such as in wardrobes, the gap beneath beds, and the cupboards under sinks and desks. On at least one occasion, a boggart was found hiding in a grandfather clock.

Famous boggarts
Famous Boggarts include the Old Boggle of Canterbury (believed by local Muggles to be a mad, cannibalistic hermit that lived in a cave; in reality a particularly small Boggart that had learnt how to make the most of echos); the Bludgeoning Boggart of Old London Town (a Boggart that had taken on the form of a murderous thug that prowled the back streets of 19th century London, but which could be reduced to a hamster with one simple incantation); and the Screaming Bogey of Strathtully (a Scottish Boggart that had fed on the fears of local Muggles to the point that it had become an elephantine black shadow with glowing white eyes, but which Lyall Lupin of the Ministry of Magic eventually trapped in a matchbox).

Lessons in Hogwarts
"That suggests that what you fear most of all is - fear. Very wise, Harry."

- Remus Lupin to Harry Potter

In 1993, Professor Remus Lupin taught his third year Defence Against the Dark Arts students about how to defend against boggarts. He took the class to the staffroom, where a boggart had moved into a wardrobe the previous afternoon. After explaining what they would need to do, Professor Lupin let the class, starting with Neville Longbottom, perform the Riddikulus charm on the boggart. When the boggart moved towards Harry Potter, Lupin drew the boggart's attention, causing it to turn into a full moon, then had Neville finish it off.

Later, Professor Lupin explained that the reason he didn't let Harry face the boggart was that he thought Harry's boggart would turn into Lord Voldemort, and he didn't want to panic the students. Harry's boggart actually took on the form of a Dementor, as he had an encounter with them on the way to Hogwarts that year and heard his mother's dying moments whenever they came near.

To help Harry, Professor Lupin taught him how to perform the Patronus, using a boggart found in Argus Filch's filing cabinet as a substitute for a real Dementor. In between the private lessons, Professor Lupin kept the boggart in a cabinet under the desk, in his office.

Triwizard Tournament's Third Task
On 24 June, 1994, Harry Potter came across a boggart in the Third Task of the Triwizard Tournament. When he first encountered it, Harry thought the boggart was actually a Dementor. He cast his Patronus, and realised that the creature was in fact a boggart when it fell back and tripped over the hem of its robes instead of being driven back, as a real dementor would have done.

Number 12 Grimmauld Place
While cleaning up 12 Grimmauld Place, the new headquarters of the reconvened Order of the Phoenix, Molly Weasley stumbled across what she thought was a boggart in a locked cabinet of a writing desk. She decided to wait for Alastor Moody to confirm it was a boggart, before opening the cabinet.

After escaping from the party being held in celebration of Ron Weasley and Hermione Granger becoming Prefects, Harry heard sobbing coming from the drawing room. When he looked inside, Harry saw Mrs. Weasley cowering against a dark wall, and the body of her son sprawled on the floor. After Molly tried the Riddikulus charm without success, Ron's body turned into the body of Bill Weasley, with his eyes wide open and empty. Molly's next attempt turned Bill's body into Arthur Weasley, with blood running down his face. The next attempts turned the boggart into Fred and George Weasley, Percy Weasley, and then Harry.

Harry shouted to Mrs. Weasley that she should leave, causing Remus Lupin and Sirius Black to run into the room, followed by Mad-Eye Moody. Lupin quickly figured out what had happened, and made the boggart turn from Harry's dead body to the full moon, then with a wave of his wand, made the orb vanish in a puff of smoke.

Unspecified
Although the precise details of this encounter are unknown, Lupin mentioned during his first lesson that he had once had an encounter with a boggart in the company of at least two other people where the boggart, attempting to frighten two people at once, turned itself into half a slug (One person was afraid of a flesh-eating slug and the other was afraid of headless corpse).

Known boggarts
"P-P-Professor McGonagall! Sh-she said I'd failed everything!"

- Hermione Granger faces her Boggart

Etymology
In Celtic mythology, a boggart (or bogart, bogan, bogle or boggle) is a household spirit, sometimes mischievous, sometimes helpful.

Behind the scenes

 * 'Boggart' is David Thewlis' favourite word/line from the series.
 * In Harry Potter and the Prisoner of Azkaban, Remus Lupin mentions that no one knows what a boggart looks like when it is alone. Mad-Eye Moody, in Harry Potter and the Order of the Phoenix, is able to determine through his "mad" eye that a creature hiding in 12 Grimmauld Place is definitely a boggart. It is assumed that Moody knew what a boggart looked like in its true form as the boggart had not seen him and shape-shifted.
 * J. K. Rowling has stated that her boggart would be the same as Molly Weasley's: her loved ones dead or alternatively herself buried alive.
 * In Harry Potter and the Prisoner of Azkaban, Remus Lupin gives Harry private lessons on how to defeat Dementors, using a Boggart as a replacement. However, when Harry faced the Boggart the first and second time, when he failed to beat it, the Boggart/Dementor had managed to make Harry hear his mother's screams. This may imply that Boggarts can gain some abilities of the thing it morphs into. It also gained its weakness as it could be defeated by Harry's Patronus.
 * Lupin's boggart, the full moon, indicated his dread of his transformations into a werewolf. However, the boggart failed to frighten him because it is not the moon itself he feared, but its effect on him.
 * The boggart may have been inspired by the "Clutterbumph" from Manxmouse, of which Harry Potter series author J.K. Rowling is a known fan. The Clutterbumph is described as follows: "...something that is not there until one imagines it. And as it is always someone different who will be doing the imagining, no two Clutterbumphs are ever exactly alike. Whatever it is that frightens one the most and that is just about the worst thing one can think of, that is what a Clutterbumph looks like."
 * In Harry Potter and the Order of the Phoenix, Molly Weasley's Boggart is shown to change from Arthur and other family members before going to Harry.
 * In an early script for the film adaptation of Harry Potter and the Prisoner of Azkaban, there were several differences from the final cut of the film. The boggarts for Neville, Ron, Harry, and Professor Lupin remained the same. Parvati's, however, would have been a vampire (specifically Count Dracula ) which would have been dressed like (presumably referring to her famous fruit hat outfit from the film The Gang's All Here) when the Boggart-Banishing Charm was used on it, with Dean having the cobra boggart Parvati has in the actual film.
 * Some changes were made in the film and video game adaptations of the series:

Appearances

 * Harry Potter and the Prisoner of Azkaban
 * Harry Potter and the Prisoner of Azkaban (film)
 * Harry Potter and the Prisoner of Azkaban (video game)
 * Harry Potter and the Goblet of Fire
 * Harry Potter and the Goblet of Fire (film)
 * Harry Potter and the Order of the Phoenix
 * Harry Potter and the Order of the Phoenix (film)
 * Harry Potter and the Deathly Hallows
 * Harry Potter and the Deathly Hallows: Part 1
 * Harry Potter and the Deathly Hallows: Part 1 (video game)
 * LEGO Harry Potter: Building the Magical World
 * LEGO Harry Potter: Characters of the Magical World
 * LEGO Harry Potter: Years 1-4
 * LEGO Harry Potter: Years 5-7
 * Harry Potter LEGO Sets