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 a shape-shifting creature 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. Because of their shape-shifting ability, no one knows what a boggart looks like when it is alone, except for Alastor Moody.

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.

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.

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 June 24, 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 Dementor would have been.

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 a silvery orb, 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 flesh-eating slug (One person was afraid of the slugs and the other was afraid of headless corpses).

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

- Hermione Granger faces her Boggart

Behind the scenes

 * In Harry Potter and the Prisoner of Azkaban, Remus Lupin mentions that no one knows what a boggart looks like when it is alone. But Mad-Eye Moody is able to in Harry Potter and the Order of the Phoenix, through his "mad" eye. It should be noted that, technically, Lupin's comment is correct, because it was made before Lupin met Moody. It is also entirely possible that Moody saw the boggart as his own worst fear, and simply knew it was a boggart, because his worst fear (whatever it may be) could not fit into a cabinet.
 * However, Lupin also acknowledges Harry's reply that "because there are so many of us, it won’t know what shape it should be" as a huge advantage when confronting boggarts since they would be confused. This implies that a boggart only take shape of whatever it thinks will frighten the person most when encountered. So Moody would have most probably seen the real form of a boggart since the boggart would not have transformed at that moment. It is also possible that Moody's worst fear was just a boggart itself.
 * 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.

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

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 Order of the Phoenix
 * Lego Harry Potter: Years 1-4

Notes and references
Épouvantard Боггарт