Muggles' Guide to Harry Potter/Magic/Dementor
|Muggles' Guide to Harry Potter - Magic|
|Features||Invisible to Muggles|
|First Appearance||Harry Potter and the Prisoner of Azkaban|
Dementors are the guards of Azkaban Prison; it is difficult to tell what their features are, because they conceal themselves completely under flowing black cloaks.
Extended Description 
It was a "cloaked figure". "Its face was completely hidden beneath its hood." When we first see it, in Harry Potter and the Prisoner of Azkaban, we do see a part of it: "a hand protruding from the cloak and it was glistening, greyish, slimy-looking, and scabbed, like something dead that had decayed in water; the thing beneath the hood, whatever it was, drew a long, slow, rattling breath, as though it were trying to suck something more than air from its surroundings."
They are dark creatures that consume human happiness, creating an ambiance of coldness, darkness, misery and despair. Because of their power to drain happiness and hope from humans, they have been designed as guards at Azkaban, where they prevent the prisoners from having the ability or desire to escape.
This creature has the property that only wizards can see it. Muggles can still feel them, though. It's only known ability, besides the happiness drain, is the ability to perform the "Dementor's Kiss", which is the act of extracting the human soul through the mouth, leaving nothing more than a living body. This process is irreversible.
They "are among the foulest creature 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. Even Muggles feel their presence, though they can't see them. Get too near a Dementor and every good feeling, every happy memory will 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'll be left with nothing but the worst experiences of your life."
The only spell we are aware of, which is able to repel these creatures, is called the Patronus charm. In Harry Potter and the Half-Blood Prince, when Harry asserts that the Patronus charm is the only way to battle Dementors, Professor Snape tells him that he is wrong; but this supposed different way of tackling Dementors is never described.
From his first exposure to them, Harry is much more sensitive to their effects than anyone else – Harry alone, of the six people present in the train compartment, passes out when the Dementor enters to search it. Harry is very much afraid that this is a display of weakness, until just before Christmas, when Professor Lupin explains that Harry is more deeply affected because Dementors work by extracting good memories, leaving nothing but the bad recollections, and he has a much grimmer history than anyone else.
In Harry Potter and the Prisoner of Azkaban, we find that Professor Dumbledore is opposed to allowing them into the school, but cannot prevent the Ministry from positioning them outside the grounds, at all the known entry points to the school. Their stated purpose at that time is to prevent Sirius Black from entering the castle, but one must wonder how the Ministry believed that would work, as Sirius had already escaped from Azkaban, so he must have some way of evading the Dementors. Possibly this is just the Ministry doing anything so that it appears to be doing something, as it so often does.
One also wonders why the Dementors would stay on as Azkaban guards, when there are so many people elsewhere in the world for them to suck emotion out of. Perhaps it was a mistake allowing so many of them to visit Hogwarts and discover exactly how thin the takings were in the prisoners at Azkaban, relative to the rest of the world. It is certainly true that they abandoned Azkaban with great alacrity in the middle of Harry Potter and the Order of the Phoenix.
While it is never stated, there is an implication that the Dementors are a created being, rather than one that occurs naturally. This, in combination with the statement in Harry Potter and the Half-Blood Prince that the Dementors are breeding, leads us to believe that they are of natural origin. Perhaps, like guard dogs, they were created from a naturally-occurring magical creature by selective breeding methods, or the traits that Wizards were interested in were magically enhanced.
- In Harry Potter and the Half-Blood Prince, the Minister for Magic attributes the gloominess of the weather to escaped Dementors breeding and multiplying. Why are Harry and Ron unable to recognize this?
Greater Picture 
Although the Dementors leave Azkaban in the middle of Harry Potter and the Half-Blood Prince to join Voldemort, one has to wonder, given their earlier actions, whose side they are on from the beginning.
All through Harry Potter and the Prisoner of Azkaban, though the Dementors are nominally seeking Sirius Black, they seem to have fixated on Harry, to the point of trying to administer the Kiss to him rather than Sirius when both of them are trapped.
At the end of Harry Potter and the Goblet of Fire, the Dementor that Cornelius Fudge has brought along with him, "for his protection," administers the Kiss to Barty Crouch Jr. before Fudge can interrogate him. If the Dementors had already allied with Voldemort, this would be sensible as it would increase the delay before Voldemort's return would be made public.
The two Dementors who attack Harry and Dudley near Privet Drive in Harry Potter and the Order of the Phoenix are apparently under orders from Dolores Umbridge, or at least so she claims near the end of that book. There is some question at that point as to whether Umbridge is actually being controlled by Voldemort, as Harry's scar did twinge at one point when she held his hand.
From the long sequence of things done by the Dementors that directly hinders the Wizarding world, one must conclude that they are more inclined to follow Voldemort than the good Wizards, and additionally that at the best of times, Wizarding control over the Dementors is very tenuous indeed.
It is a shame, perhaps, that Snape never elucidates his comment about there being a better spell to eliminate Dementors. If they are breeding, as Fudge says at the beginning of Harry Potter and the Half-Blood Prince, then they will likely need to have their numbers cut down. As it is, we don't know if there is any method of effectively doing that.