Azkaban

Azkaban is a wizarding prison located on an island in the middle of the North Sea. This prison is only used by the United Kingdom, like Nurmengard. Using certain Charms, the prison is hidden from the muggle world, and is unplottable. It is widely regarded as a horrible place, given that its guards, Dementors, cause great suffering to those in their proximity. Most prisoners eventually go insane, and slowly die away.