Dementor

Dementors are horrific wraith-like creatures that appear in the Harry Potter books and film series. These nightmarish monsters have been classed by the Ministry of Magic as "non-beings" due to their amortal state, meaning that they are neither capable of dying nor have they ever been alive. Dementors feed on positive human emotions, draining all feelings of joy and happiness from anyone in their proximity. They reside within the island fortress of Azkaban, serving to guard the criminals of the Wizarding World.

Appearance
A Dementor closely resembles a wraith, appearing as a tall, lean humanoid figure completely covered in pitch-black robes. The majority of the Dementor's body cannot be seen, though its hands and occasionally its mouth are visible and bear deathly-grey skin.

Dementors are shown to both glide across terrain or fly freely. They seem to exude cold, generating a chilling mist wherever they go and causing their surroundings to freeze under a veneer of frost. Dementors do not have eyes and cannot see, though they do not require eyesight to hunt humans as they can sense the emotions of their prey.

Behaviour
Dementors seem to suck all the light and happiness from the air as they draw long rattling breaths, and cause darkness to close in everywhere and the victim to feel incredibly cold. They can glide toward a victim without sound, and they are sentient enough to taste and sense fear, being drawn to it and the promise of positive memories that they can feed off of. They are also intelligent enough to be greedy: they obeyed the Ministry of Magic for years because, in guarding Azkaban, they were provided with sustenance of any remaining hope or happiness in the prisoners. 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 could communicate what they had heard Sirius Black saying in his sleep to the Ministry in 1993, implying that they can speak or otherwise communicate with wizards. Albus Dumbledore also describes Dementors as vicious creatures and that they won't ever distinguish between those they are hunting and those that are in their way. He also informs the students to give Dementors no reason to harm them, saying that forgiveness is not in a Dementor's nature.

Outside of Azkaban, Dementors appear to hunt for prey in groups of variable size, from as small as two to as large as greater than twenty. They appear to attack by first surrounding their targets, cutting off any escape, then descending upon them all at once to feed.

Abilities
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. After spending only a few months there, Rubeus Hagrid claims he wished he would die in his sleep.

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 sneaked into Azkaban disguised as Bartemius Crouch Jr using Polyjuice Potion, or Barty Crouch Jr being sneaked out disguised as his mother, since both were ill nearly to the point of death, as divulged by Bartemius Crouch Jr under the influence of Veritaserum in 1995 (though it may be that they simply did not care who they were sucking happiness from so long as they were "fed"). They can also not distinguish between humans and animals very easily; Sirius Black, an Animagus, escaped from Azkaban in his dog form. When he became his dog form during imprisonment, the Dementors thought he was losing his mind as his emotions were less human in his animagus form.

Although Muggles cannot 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 cannot 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 capable of physical labour, such as grave-digging. Also, Sirius escaped when the Dementors came to give food, another sort of labour.

It is stated by Remus Lupin that, if a wizard remains in close contact with Dementors for an extended period of time, a Dementor can drain a wizard of his powers. This fact is not expounded upon, so the truth to this statement is unknown. If this is the case, however, it would make Dementors the only known being to permanently render a wizard without magic. Lupin might also mean this metaphorically, as severe depression, which is caused by prolonged contact with Dementors, can be really detrimental to a wizard's ability to cast spells.

In addition to feeding on positive emotions, Dementors can perform their last and most dangerous ability: 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 for Magic. 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. It is also possible that the soul is trapped inside the Dementor, thus the victim in unable to pass on. This is probably what "worse than death" is.

The only known named victim of the Kiss is Barty Crouch Jr. From 1994 to 1995 Crouch had been using polyjuice potion to pose as Alastor Moody, and had been teaching at Hogwarts in his stead; he was detained at the castle, awaiting trial, after his ploy was discovered, and it emerged that he had been working in secret for Voldemort. He was subjected to the punishment by a Dementor accompanying Cornelius Fudge as bodyguard. The unfortunate 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
No one has ever demonstrated the ability to kill a Dementor, by Avada Kedavra or otherwise, 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 the very difficult Patronus Charm. The charm summons a Patronus, a 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. With sufficient ability, a single wizard can hold off dozens, if not hundreds, of Dementors with a single corporeal Patronus, as Harry Potter demonstrated during his third year at Hogwarts when he drove off a horde of Dementors seeking to consume Hermione Granger, Sirius Black, and Harry's past selves.

As the Patronus is not alive, the Dementor cannot feed on it. 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 is 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.