Azkaban

Azkaban is the wizarding prison of Great Britain, a fortress standing alone on an island in the North Sea. The name first surfaces in Harry Potter and the Chamber of Secrets as a threat that frightens grown wizards, and Harry Potter and the Prisoner of Azkaban brings the place itself into focus. Its guards are dementors, hooded creatures that feed on happiness until prisoners are left alone with their worst memories, and most inmates lose the will to do much of anything at all. No wall needs to be high when the guards can drain away the hope an escape requires, and the wizarding world regards getting out of Azkaban as impossible. A sentence there is the heaviest punishment wizarding law can impose, and the mere mention of the prison ends most arguments.