Dementor

Dementor
Dementors in the Prisoner of Azkaban film.
Species	Dark Creature
Common Habitats	Azkaban prison (guards)
Affiliation	Azkaban Voldemort (Temporarily)

Dementors are cruel creatures who suck the happiness out of anyone they please and inject depression and sadness into the areas they wander. They can permanently suck people's souls and leave them in a stage described to be much worse than death. They were once used as guards for the Prison of Azkaban and constantly monitored the cells of the prison; however, during the events of the Half-Blood Prince and the Deathly Hallows, the Dementors abandoned their posts at Azkaban and joined the ranks of Voldemort, who could offer them more victims. The Ministry of Magic stripped them from their position as guards after the fall of Voldemort.

Dementors cannot be killed, but can be temporarily driven away using the Patronus charm, a difficult charm that is revolved around happiness.

J.K. Rowling stated that the Dementors were inspired by her own depression before her success with the Harry Potter series. She said that she had felt empty, like she'd never be cheerful again, and that is was very different from sadness, mirroring the effects of the Dementors' presence.

Appearance

Dementors are described to be human-like creatures in gray, ragged cloaks with their hoods up, and can float freely. Their hoods fall and their hole-shaped mouths open when they are giving a Dementor's Kiss, a process in which Dementors suck the soul out of a person and leave them in a soulless stage.

History of Encounters with Dementors

Harry Potter

Harry first encountered the Dementors on his train journey to Hogwarts in his third year, when they were searching for Sirius Black, who had escaped from Azkaban Prison. He was later attacked by Dementors during a Quidditch match against Hufflepuff, causing him to fall off his broom mid-flight. Professor Lupin explained to Harry that the Dementors were drawn to him because of his traumatic past, which made him more vulnerable to their effects, and then agreed to teach to him the Patronus Charm, using a Boggart to simulate a Dementor. Harry was unable to produce a corporeal Patronus, however.
After Harry met and learned the truth about Sirius, that he was innocent, the Dementors found and attacked them. The Dementors almost succeeded in extracting Sirius' soul, but were driven away by a corporeal Patronus in the form of a stag. This Patronus turns out to be Harry's own, as he and Hermione went back in time using the Time-Turner Hermione was given at the start of the year by Professor McGonagall to save Sirius. It took the form of a stag because it was his father's Patronus and Animagus form.

Harry next encountered Dementors in the Order of the Phoenix, before he returned to Hogwarts. He and his cousin, Dudley, were attacked by Dementors, which Harry drove away using his Patronus. The Ministry of Magic threatened to expel Harry for using magic in front of a muggle, but he was released of all charges after his hearing. It is later revealed that Dolores Umbridge released the Dementors which attacked Harry and Dudley, in order to have Harry expelled.

In Harry Potter and the Deathly Hallows, when Harry, Ron and Hermione infiltrated the Ministry under the control of Voldemort, the Dementors were used to escort convicted muggle-borns and torture them during their interrogation.
The Dementors later appeared during the Battle of Hogwarts, fighting for Voldemort, along with many other magical creatures.

Sirius Black

Sirius had many close encounters with Dementors, having been a prisoner in Azkaban for twelve years. He managed to escape, being the first person to do so. He did this by transforming into his Animagus form, a black, shaggy dog. Dementors are unable to sense the less complex emotions of animals, and so Sirius was able to escape in this form. Later that year, he was finally found by the Dementors at Hogwarts, but was saved by Harry casting his Patronus.