A dementor is an utterly foul fictional being, the worst creature J. K. Rowling's Harry Potter book series has to offer. They guard the extremely high-security wizard prison of Azkaban.

In appearance, dementors are humanoid figures approximately eight feet in height covered in dark, hooded cloaks which reveal only grey, scabby, decaying hands. They infest the darkest, dankest places and do not breed; instead, they grow like fungi in these habitats (according to the author). Their movie counterparts are considerably shorter, and fly instead of gliding.

The creatures have no eyes. They sense and feed on the positive emotions of human beings, forcing them to relive their worst memories. The very presence of one makes the surrounding atmosphere grow cold and dark, and the effects are cumulative with the amount of dementors present. Those kept in the company of a dementor for too long are often driven insane, which is the main source of Azkaban's well-deserved horrible reputation. Their source of sustenance allows the Ministry of Magic to reluctantly employ them; dementors are at least intelligent enough to be greedy, and to obey as long as doing so provides them with a bigger feast. Dementors are invisible to Muggles, but affect them the same way.

The only way to shield oneself from dementors is to use the difficult Patronus Charm to drive them away. The charm summons a Patronus, a sort of physical manifestation of good will, which has no despair for dementors to thrive on. Chocolate is an effective first aid to mild cases of contact.

Unfortunately, the feeding is not the worst part. Dementors have an ultimate weapon known as a Dementor's Kiss which quite literally sucks out a person's soul (the mouth of a dementor is not a pretty sight). The victim is left an empty shell, yet not quite dead -just a soulless, gibbering person, incapable of a single thought. (Compare brain death.) The Ministry of Magic seems to occasionally allow this as a punishment.

Dementors in the narrative

Harry Potter first encountered dementors during the school year of 1993-1994, when they were sent to guard Hogwarts School of Witchcraft and Wizardry against Sirius Black, who had recently escaped Azkaban Prison. Harry, whenever he got near one, was forced to relive his worst memory: hearing the last moments of his parents before they were murdered by Lord Voldemort. To overcome the dementors, Harry asked the Defence Against the Dark Arts professor, Remus Lupin, for assistance. Lupin taught Harry the Patronus Charm.

At the end of this year, a dementor disregarding its orders for the sake of feeding tried to administer the Kiss on Harry, but he used his Patronus to drive it away. Harry's Patronus took the form of a stag for the first time as he mastered the spell at a remarkably young age.

His second encounter was in 1995, in Privet Drive, when he and his cousin Dudley Dursley were chased by two dementors. He was able to successfully use the Patronus Charm and was subsequently rescued by a Squib named Arabella Figg.

The dementors served the Ministry of Magic as the guards of Azkaban because it allowed them to feast on their human victims. However, at the end of Harry Potter and the Order of the Phoenix, the dementors of Azkaban mass revolt against their employers to join Lord Voldemort as he can provide them with more humans to feast upon.