The guardians of Azkaban, the Dementors are soul-sucking wraiths that live off of people’s worst fears. Nightmarish in appearance and capable of reducing a human being to an empty shell by kissing them, the Dementors are perhaps the most sinister magical creatures in Rowling’s world.
The concept for the Dementors, says Rowling, comes from her experiences with depression, and given their description, that’s easy to understand. Harry, upon first encountering one at the beginning of Prisoner of Azkaban, says he felt as though he’d never be happy again. Ron and Hermione, though not as affected, shared similar feelings. The film version further emphasizes the Dementors’ power by showing water freezing, plants withering, and glass fogging over.
Rowling says that Dementors grow like fungi in dark, dank places, and create a chilly, thick fog. Virtually nothing is revealed about them regarding language, culture, society, or even if there are male and female Dementors, they are feared and avoided by the wizarding world for their ability to drive a person insane. It is this ability, plus the Dementor’s Kiss, that leads to the Ministry of Magic engaging them as guards for Azkaban, a task they undertake presumably because doing so gives them a stable food supply.
Dumbledore protests this arrangement, as well as their presence at Hogwarts. And when Lord Voldemort returns to power at the end of Goblet of Fire, the Dementors side with him due to his promise of more people to feed upon. They abandon their posts at Azkaban and allow for a mass escape.
The name is obvious enough: Dementor is reminiscent of the word “demented”, which Rowling herself must have realized when she named the opening chapter of Order of the Phoenix “Dudley Demented”. Demented comes from the earlier verb dement, meaning to “drive mad”. This in turn comes from the French dément or the Latin adjective demens, meaning insane, and literally meaning “having the mind removed”, since mens is the Latin word for mind.
These associations carry over nicely in the various European translations of Harry Potter, but don’t work well at all in other languages.
The Spanish and German, among other languages, keep the same name, with the German capitalized because in German nouns are capitalized. The French détraqueur comes from the verb détraquer, meaning to put out of order, be unsettled, or be unstable, and the adjective détraqué, meaning broken down, unhinged, or unbalanced. It is a good choice for capturing the idea of a Dementor.
The Japanese characters, annotated with subscript indicating that they should be read “dementah” rather than their usual “kyukonki”, means soul-sucking demon, which is literally accurate but entirely awkward. Unfortunately, the Japanese translation of Prisoner of Azkaban was so poor that the translator had to apologize publicly for it.
The film version of the Dementors bears a striking resemblance to the Ringwraiths portrayed in The Lord of the Rings movies. Both are magical beings swathed in layer upon layer of dark, moudly gray and black cloth. But while the Ringwraiths walk and ride horses, the Dementors float or fly. We see their hands, but never their feet, if they have any. Also, while there are, by definition, nine Ringwraiths in The Lord of the Rings, the Dementors seem to be a species, and their total population is not known.
The Dementors also resemble the classic version of the Grim Reaper, minus the scythe. Whether the Ghost of Christmas Future, Death in the Discworld series, or even Homer Simpson in a Halloween special, the Grim Reaper is consistently a tall figure swathed or cloaked in black and gray, with bones for hands, and at most a skull for a face. This image is old, visible on medieval manuscripts, and a likely choice for any death-dealing creature in a fantasy world.
The only other useful piece of information we are told is how to defend oneself. The Patronus Charm is, as Harry learns from Professor Lupin in Prisoner of Azkaban, the way to drive back a Dementor, and a sufficiently powerful Patronus can hold off a horde of them, as Harry proves late in that book.