Dementor

"Dementors are among the foulest creatures that walk this earth. They infest the darkest, filthiest places, they glory in decay and despair, they drain peace, hope, and happiness out of the air around them... Get too near a Dementor and every good feeling, every happy memory will be sucked out of you. If it can, the Dementor will feed on you long enough to reduce you to something like itself...soulless and evil. You will be left with nothing but the worst experiences of your life."

- Remus Lupin to Harry Potter

A Dementor is a Dark creature, considered one of the foulest to inhabit the world. Dementors feed off human happiness, and thus cause depression and despair to anyone near them. They can also consume a person's soul, leaving their victims in a permanent, and thus are often referred to as soul-sucking fiends. The Ministry of Magic employed Dementors as the guardians of Azkaban until mid-1996, when Lord Voldemort was sighted in the Ministry, and their defection to his side was realised. The Dementors supposedly led the Death Eaters and Voldemort into the Ministry of Magic. After the end of the Second Wizarding War in 1998, the Ministry was reformed, and Minister for Magic Kingsley Shacklebolt ensured that they were not used by the government again.

Dementors hold no true loyalty, except to whomever can provide them with the most people to feed off. They cannot not be destroyed, though their numbers can be limited if the conditions in which they multiply are reduced, implying that they do die off eventually.

Appearance
Dementors have a humanoid shape, approximately three metres, or 10 feet, high, and are covered in dark hooded cloaks of long, ripped cloth. The creatures have grey, decayed-looking bodies and their faces have nothing but a large hole where the mouth should be, which is used for sucking the soul out of the victim in a process called the Dementor's Kiss, which leaves victims in a state generally considered worse than death. Unlike other creatures, who move through some means of physical support, Dementors are able to fly freely.

Nature
"They sort of freeze your insides, don't they?"

- Fred Weasley on proximity to Dementors.

Dementors are at least somewhat sentient, and intelligent enough to be greedy: they obeyed the Ministry of Magic for years because, in guarding Azkaban, they were provided with sustenance. They can understand and follow at least simple instructions, which allowed a Dementor to act as a bodyguard to Minister for Magic Cornelius Fudge in 1995, and for them to escort prisoners in and out of the Muggle-Born Registration Commission courtroom in 1997. They were able to communicate what they had heard Sirius Black saying in his sleep to the Ministry in 1993, implying that they can relay thoughts of their own to wizards through some unknown form of communication, though the extent to which Dementors and and wizards can communicate is unclear. It is possible that Dementors react to emotion and/or thought, rather than to spoken words, though it's also possible they can hear and understand speech.

Abilities
"They don't need walls and water to keep the prisoners in, not when they're trapped inside their own heads, incapable of a single cheerful thought. Most go mad within weeks."

- Remus Lupin on Azkaban prisoners' long-term exposure to Dementors

Being blind, Dementors sense and feed on the positive emotions of human beings in order to survive, forcing their victims to relive their worst memories over and over again. The very presence of a Dementor can make the victim's surrounding atmosphere grow cold and dark, and as the number of Dementors increase, so do the effects. Those that are kept in the company of a Dementor for too long tend to become depressed, and are often driven insane, which was the main source of Azkaban's well-deserved horrible reputation when they still guarded its prisoners.

Dementors rely fully on their ability to sense emotion and physical and/or mental health to track the movements of the prisoners in their care, and are generally unable to distinguish one person from another if both are in similar physical or mental condition. They were unable to identify Mrs. Crouch being snuck into Azkaban disguised as Bartemius Crouch Jr. using Polyjuice Potion, or Barty Crouch Jr. being snuck out disguised as his mother, since both were ill, as divulged by Bartemius Crouch Jr. under the influence of Veritaserum in 1995.

Although Muggles can't see Dementors, they are affected in a way similar to wizards and witches, becoming depressed when near them. While at least one Squib, Arabella Figg, has claimed to see a Dementor, the truth is that Squibs can not see them, but have enough magical knowledge to identify their effects.

After Bartemius Crouch Jr.'s mother died in Azkaban, Dementors buried her body, which shows that Dementors are also capable of physical labor, such as grave-digging.

Dementor's Kiss
"There's no chance at all of recovery. You'll just... exist. As an empty shell. And your soul is gone forever…lost."

- Remus Lupin on the Dementor's Kiss

In addition to feeding on positive emotions, Dementors can perform what is known as the Dementor's Kiss, in which a Dementor latches its mouth onto a victim's and sucks out the person's soul. The victim is left as an empty shell, incapable of thought and with no possibility of recovery. It is believed that existing after a Dementor's Kiss is worse than death: as a person's soul is their true self, to be "kissed" by a Dementor is to cease to exist, and yet also remain. The Ministry of Magic occasionally allowed this as a punishment, before Kingsley Shacklebolt became Minister.

Muggles who have had their souls sucked out in a Dementor's attack are often mistakenly be thought to be suffering from the medical condition known as by Muggle doctors, since after the 'kiss', the victim would no longer experience any brain activity despite no damage to brain actually having occur. It is unknown what effect a Dementor's Kiss would have on a person who had split his or her soul through use of a Horcrux.

The only recorded Kiss occurred in 1995; Barty Crouch Jr., who had been working in secret for Voldemort, and was restrained at Hogwarts awaiting trial. His soul was sucked out by the Dementor accompanying Cornelius Fudge as a bodyguard. The terrible consequence of this act was that it rendered Crouch unable to testify about his master's return, allowing Voldemort almost a full year to gather strength while the Ministry continued in its denial.

Protection from Dementors
"A Patronus is a kind of positive force, and for the wizard who can conjure one, it works something like a shield, with the Dementor feeding on it, rather than him."

- Remus Lupin teaching Harry Potter the Patronus

No one has ever demonstrated the ability to kill a Dementor, implying that they can not be killed through physical means, but can only be driven away or temporarily kept at bay. One of the few ways to shield oneself from Dementors is by using of the difficult Patronus Charm to fend them off. The charm summons a Patronus, the magical manifestation of good will and happiness, providing varied levels of protection against the Dementors' influence, based on the caster's strength as a wizard. Only when summoned by an experienced caster will the Patronus take the form of an animal significant in some way to the individual. While the lower level Patronus was more amorphous and ephemeral, corporeal Patronuses chase down Dementors and force them to flee the vicinity. An incorporeal Patronus only slows a Dementor down, as if the creature is walking through quicksand, and tends not to last long.

Since Dementors can not properly sense the less complex thoughts and emotions of a person when he or she takes on an Animagus form, doing so allowed some, such as Sirius Black, to temporarily avoid feeling the effects of the dementors' presence.

Though clearly not a common method of repelling dementors, the enigmatic form people take after being brought back by the Resurrection Stone appears to function much like a Patronus, as the "resurrected" forms of Sirius Black, Remus Lupin and James and Lily Potter were able to shield Harry Potter from the Dementors had stationed in the Forbidden Forest in 1998.

There may be another, more common way of repelling a Dementor, aside from the Patronus Charm, as Harry Potter disagreed with Professor Snape on the best method for confronting one when it was the subject of a report in his sixth year. This implies that there are indeed other methods, which seems even more likely given that most Dark Wizards are apparently unable (or unwilling) to produce a Patronus.

Chocolate is an effective first-aid to mild cases of contact, given to a victim to help regain their strength after an encounter with Dementors.

Separation from the Ministry
The Dementors served the Ministry of Magic as the guards of Azkaban, as it allowed them to feed on the emotions of the prisoners. However, by July 1996, most of the Dementors of Azkaban staged a mass revolt against their employers to join Lord Voldemort, who offered them more victims. This aided the 1996 and 1997 escapes of Death Eaters from Azkaban.

By mid-1996, the Dementors, who had joined Voldemort, were multiplying, thus causing an "unseasonal" July mist. Cornelius Fudge also admitted they were factors in the British Prime Minister's recent decline in terms of public approval, as they were making people less happy in general.

When Kingsley Shacklebolt became Minister for Magic after the end of the Second Wizarding War in 1998, Dementors were no longer used by the Ministry of Magic, presumably because they were deemed untrustworthy and exposing people to their effects is considered inhumane.

Harry Potter's encounters with Dementors
"When they get near me -- I can hear Voldemort murdering my mum."

- Harry Potter on the Dementors' effect on him

1993-1994
Harry Potter first encountered Dementors during the 1993–1994 school year, when they were sent to guard Hogwarts School of Witchcraft and Wizardry against Sirius Black, who had recently escaped from Azkaban. Harry, whenever he got near one, was forced to relive his worst memory: hearing the last moments of his parents' lives before they were murdered by Lord Voldemort. The first of these encounters was on the Hogwarts Express, during which he was protected by the new Defence Against the Dark Arts professor, Remus Lupin. While the Dementors were at Hogwarts, Headmaster Albus Dumbledore warned the students not to give the Dementors any reason to harm them.

Harry endured a second encounter with a group of Dementors during a Quidditch match, which caused him to lose consciousness and suffer a fall from his broom. He was that time rescued by Dumbledore, who slowed Harry's fall and then drove the Dementors away with a Patronus. They seemed to be particularly attracted to Harry because, due to his miserable childhood and many bad memories, he was particularly vulnerable to their influence.

To overcome the Dementors, Harry asked Professor Lupin for assistance. Lupin taught Harry the Patronus Charm. Harry's Patronus took the form of a stag, just was both the same form his father's Patronus took, and the form his father himself took as an Animagus. Harry initially failed to produce a fully formed Patronus when he and Sirius were attacked by a group of Dementors in the Forbidden Forest; the two were saved by the intervention of a powerful and fully formed Patronus, which had in fact been conjured by Harry, who travelled back in time with Hermione Granger by use of her Time-Turner (this technically counts as both Harry's third and fourth encounters with the Dementors, since, due to his time traveling, he experienced this same encounter twice). Because the Dementors tried to use the Dementor's Kiss on Harry during this encounter in the Forest, it was enough to convince the Minister for Magic Cornelius Fudge to have the Dementors removed from Hogwarts at the end of the school year.

1995
Harry's fifth encounter was in an alley near his home on Privet Drive, when he and his cousin Dudley Dursley were ambushed by two Dementors, sent secretly and illegally by Dolores Umbridge. He was able to successfully use the Patronus Charm and was subsequently rescued by a Squib named Arabella Figg. Harry was put on trial for his use of underage magic, but ultimately not punished, as it recognised as having been self-defence.

1997
While Harry was disguised as Albert Runcorn he encountered the Dementors in the Muggle-Born Registration Commission court room, where they were used to take away convicted Muggle-borns, following interrogation.

1998
During the Battle of Hogwarts, the Dementors fought on the side of Lord Voldemort and the Death Eaters. Harry, Hermione Granger, and Ron Weasley were attacked by large numbers of them, and due to the horrific events they had experienced and witnessed, including the recent death of Fred Weasley, they all had difficulty summoning their Patronuses. Seamus Finnigan, Luna Lovegood, and Ernie Macmillan drove them off with the charm.

Etymology

 * The name Dementors likely comes from the words dement, "to make insane", and tormentor, "one who afflicts with great suffering", which would make the definition of the word "one who inflicts insanity".

Behind the scenes

 * J. K. Rowling has revealed that the inspiration for Dementors came from her bout with severe depression prior to her phenomenal success. She described as an "absence of being able to envisage that you will ever be cheerful again. The absence of hope. That very deadened feeling, which is so very different from feeling sad."
 * In an attempt to sabotage Harry during a Quidditch match in 1993, Draco Malfoy, Vincent Crabbe, Gregory Goyle, and Marcus Flint dressed up as dementors to scare him. Malfoy was standing on Goyle's shoulders. Considering how tall dementors are (they are described in Prisoner of Azkaban as reaching from the floor to the ceiling on the Hogwarts Express), the foursome appear to account for two dementors, when just a few pages earlier, Harry saw three dementors. However, their attempt failed when Harry cast a Patronus Charm at them. As punishment Professor McGonagall gave them all detention and deducted 50 points from Slytherin, plus a serious discussion with Headmaster Albus Dumbledore.
 * Animatronic dementors appear in Harry Potter and the Forbidden Journey at The Wizarding World of Harry Potter, chasing the riders' flying bench through the Chamber of Secrets and around the skeleton of the Basilisk. The Dark Mark appears along with them, suggesting that a Death Eater summoned them to Hogwarts.
 * According to W.O.M.B.A.T., Dementors may be unknown in tropical regions.
 * In Lego Harry Potter: Years 1-4, a large Dementor serves as the final boss of Year 3.
 * J.K. Rowling has said that Dementors breed like fungi.

Foreign words for Dementor

 * Chinese 摄魂怪 = "devil strange"
 * Finnish "ankeuttaja", from "ankeus" = "drearyness"
 * French "détraqueur" = "that which makes (things) go wrong or break down"
 * Hebrew "סוהרסן", from "סוהר" = "prison"
 * Japanese 吸魂鬼 = "intake/suction soul demon"
 * Vietnamese "giám ngục Azkaban" = "watcher of Azkaban"
 * Czech "mozkomor" = "plague of the soul"
 * Indonesian "iblis" = "demon of death and happiness remover"
 * Turkish "Ruh Emici" = Ruh means soul and verb emmek means 'to suck', it means Soul Suckers
 * Thai "ผู้คุมวิญญาณ" (pronounced "Poo-Khum-Win-Yarn") = Soul Warden

Appearances

 * Harry Potter and the Chamber of Secrets (video game)
 * 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
 * Lego Harry Potter: Years 1-4
 * Harry Potter and the Order of the Phoenix
 * Harry Potter and the Order of the Phoenix (film)
 * Harry Potter and the Order of the Phoenix (video game)
 * Harry Potter and the Half-Blood Prince
 * Harry Potter and the Deathly Hallows
 * Harry Potter and the Deathly Hallows: Part 1
 * Harry Potter and the Deathly Hallows: Part 1 (video game)
 * Harry Potter and the Deathly Hallows: Part 2

Notes and references
Détraqueur Dementor Дементор סוהרסן Ankeuttaja