A Dementor is a gliding, wraith-like dark creature (considered one of the foulest to inhabit the world) employed by the British Ministry of Magic as the guard of Azkaban Prison. The Dementors feed upon 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". They are known to leave a person as an 'empty-shell'.
The Dementors of Azkaban were under the employ of the Ministry until mid-1996, when Lord Voldemort was sighted in the Ministry, and their defection to the Dark Lord's cause became undeniable. The Dementors supposedly led the Death Eaters and Voldemort into the Ministry of Magic. 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, presumably because of their actions in support of Voldemort and due to ethical reforms in the Ministry and Wizarding society at large .
There are certain defences 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 eventually.
