Azkaban. Universally feared as the worst possible punishment for any and all magical crime, Azkaban is the only known wizard prison.

So far as we know, there is no crime in the wizarding world severe enough to merit capital punishment, but to those wizards who run afoul of the law, a death sentence might be preferable to a prolonged stretch in Azkaban. Certainly for those crimes severe enough to merit a life sentence, such as the use of an unforgivable curse on a fellow human being, death would be quicker and less painful. Azkaban is feared as the worst possible punishment, due mainly to the prison's guards, the loathesome Dementors.

The Dementors of Azkaban are some of the most horrific magical creatures in existence. Tall figures, shrouded in black, their skin is rarely visible, but when revealed is seen to be glistening, greyish, slimy and scabbed. Foul in every way, Dementors live in darkness and filth, and glory in decay and despair. Able to drain an area of peace, hope and happiness, they can suck every good feeling and happy memory out of a person. The prisoners of Azkaban are forced to relive all of the worst moments of their lives, and will eventually forget that they were ever happy and lose all sense of hope.

Notable inmates of Azkaban are the erroneously imprisoned Sirius Black, the first inmate ever to escape the prison and Rubeus Hagrid, who was incarcerated for a brief period during Harry's second year due to suspicions concerning the Chamber of Secrets. Also among the many prisoners who must have been left mostly unguarded since the Dementors joined the resurrected Voldemort were a number of Death Eaters, who have of course left to rejoin their Lord.