Index · creature · book 3
Dementors
Creatures that feed on happiness and leave a fog of despair. The state uses them as prison guards.
Dementors are a climate with a hood. Cold arrives first. Memory goes thin. A kiss, in the grim vocabulary of the books, is not affection — it is the removal of a soul. The ministry employs them at Azkaban, which tells you what that government will tolerate when it is afraid.
The defense associated with them is the Patronus charm. Canon fact: dementors guard the prison and can be ordered elsewhere. Whether they should exist as a sentence is a question the novels eventually stop dodging.