Island in the North Sea
Azkaban, also known as Azkaban Prison, is a prison on an island in the middle of the North Sea. It serves the magical community of Great Britain. Using certain Charms, the prison is hidden from the muggle world, and is unplottable. It is thought to have undetectable extension charms placed on it (or similar types of magic) to make it bigger on the inside, as the prison seems to serve the whole of wizarding Britain. Although other wizarding prisons exist, such as Nurmengard (which was used exclusively to hold its creator Gellert Grindelwald), Azkaban is the only known fortress to be the "official" holding ground for wizarding criminals.
Azkaban gained a reputation as a horrible place, mostly due to the prison's guards:Dementors, foul creatures that suck the happiness out of the victim and cause great suffering to those near them. Because of this, most of the prisoners eventually went insane and slowly died under the Dementors' influence. Since 1717, using any of the three Unforgivable Curses on another human being has carried a punishment of a life sentence in Azkaban (unless there is sufficient evidence that the caster did so under the influence of the Imperius Curse, or legal exceptions made by the Ministry of Magic as in the case of Aurors in the First Wizarding War).