Dementor

A Dementor is a fictional foul, dark creature; empathic, soul-sucking Fiend; that guards the wizard prison of Azkaban, in the Harry Potter series of books by J. K. Rowling. Dementors wear dark, hooded cloaks, which reveal only grey, scabby, decaying hands. Dementors are also very tall, approximately eight foot in height. They infest the darkest, dankest places.

Dementors have no eyes. They sense and feed on the positive emotions of human beings, forcing them to relive their worst memory - the same as depression. A Dementor's presence makes the surrounding atmosphere grow cold and dark. Eventually, if kept in the company of a Dementor for too long, one will be driven insane by wallowing in one's own sadness and horror. This is why Azkaban is such a terrible place. The only way to overcome Dementors is by using the Patronus Charm, a very advanced and difficult charm that drives them away. This charm summons a Patronus, a sort of physical manifestation of good will, but because it has no despair (as a human does) it is unharmed by the Dementor's feeding.

Aside from the process described above Dementors have one weapon, a truly terrible weapon, to which no cure is known. This is known as the Dementor's kiss. The power of the kiss is to suck up a person's soul, leaving them an empty shell, yet not quite dead -just a soulless, gibbering person, incapable of a single thought. Compare brain death.

Warning: Plot details follow.

Harry Potter first encountered the 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 that year 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 only way to drive away the Dementors, the Patronus Charm.

In this year, a Dementor tried to administer the Kiss on Harry, but he used his Patronus to drive him away. Harry's Patronus took the form of a stag, the animal which his father became when he transformed as an animagus.

His second encounter was in 1995, in Privet Drive, when he was chased by two Dementors with his cousin, Dudley Dursley. He was, however, rescued by a squib named Arabella Figg, after Harry used his Patronus Charm.

At the end of Harry Potter and the Order of the Phoenix, the Dementors of Azkaban mass revolt against their employers (the Ministry of Magic) to join Lord Voldemort.

Notes- in the movie dementors fly rather than glide, and are also much shorter than in the books.