Dementor! Dementor! (in Different Languages)
5,610A Dementor is a gliding, wraithlike Dark creature, widely considered to be one of the foulest to inhabit the wizarding world. Dementors feed on human happiness and thus generate feelings of depression and despair in any person in close proximity to 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", rendering a person an 'empty-shell'. Dementors were closely associated with Azkaban, as they were formerly employed by the British Ministry of Magic as the prison guards, and were not known to permanently inhabit any other location.
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. After the end of the Second Wizarding War in 1998, the Ministry was re-formed, and newly-appointed Minister for Magic Kingsley Shacklebolt ensured that they were not used by the government again, presumably due to their changing allegiance and 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 whoever can provide them with the most people to feed on. They seemingly cannot be destroyed, though their numbers can be limited if the conditions in which they multiply are prevented from forming, implying that they do die off eventually.