Dementor
A Dementor is a non-being and 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 vegetative state, and thus are often referred to as "soul-sucking fiends" and are known to leave a person as an "empty-shell."
The Ministry of Magic employed Dementors as the guards of Azkaban prison, until mid-1996, when Lord Voldemort was sighted in the Ministry, and their defection to his side was realised. 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.
There are certain defenses one can use against Dementors, specifically the Patronus Charm. Dementors hold no true loyalty, except to whomever can provide them with the most people to feed off. They cannot be destroyed, though their numbers can be limited if the conditions in which they multiply are reduced, implying that they do die off, or as they are amortal dissipate, eventually.
From Harry Potter Wiki:Dementors
Description
Appearance
Dementors have a humanoid shape, approximately three metres, or 10 feet high, and are covered in dark hooded cloaks of long, ripped black cloth, making them closely resemble wraiths. Its body is greyed and decayed looking, and its breath sounds rattling, like it is trying to "suck more than air" out of a room. Its hands are "glistening, greyish, slimy-looking, and scabbed". They seem to exude cold.
A Dementor's face consists of empty eye sockets, covered with scabby skin. There is also a gaping 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, leaving victims in a state generally considered worse than death. The Dementor pulls back its hood and sucks out his or her soul, leaving the person an empty shell, alive but completely, irretrievably "gone." Unlike other creatures, who move through some means of physical support, Dementors are able to fly freely.
Dementors are also known to be blind, as they do not have any eyes. However, they have the ability to sense whether a presence is near them or not. This is because they sense the victim's emotions.
Nature and Behavior
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 it 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 are able to communicate with wizards.
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.
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 can also not distinguish between humans and animals very easily.
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.
Dementor's Kiss
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. Muggles would describe it as a permanent vegetative state.
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, most notably for Azkaban escapees, before Kingsley Shacklebolt became Minister for Magic.
Dementor Exposure
Treatment for dementor exposure is fairly simple, but it has varied success rates depending on the severity of the exposure to the dementors. See full article on Dementor Exposure
Protection from Dementors
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 use 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. The Dementor is unable to 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 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.
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.
Dark Wizards are not able to produce Patronuses. However, they do not need to do so in the first place, as they are like-minded in evil.
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, all of the Dementors of Azkaban staged a mass revolt against their employers to join Lord Voldemort, who offered them more victims
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, because they were deemed untrustworthy and exposing people to their effects is considered inhumane.
At Absit Omen
Dementors were once used as part of the guard at Azkaban until the ministry severed ties after the Dementors sided with Voldemort. A speacial draught that copies the effects of a dementor's presence was made by the Department of Mysteries, using an essence of the dementor.
One historical theory into the early days of Azkaban prision was that prisoners were used as test subjects in a form of soul splitting astral projection, theoretically caused by interrupting a dementor's kiss and storing the soul in Muhra Glass. Discussed within this thread [1]
