J.K. Rowling’s fictional prison for dangerous wizards is Azkaban.
Azkaban is an island prison in the North Sea used by the British wizarding authorities. It is guarded by Dementors, beings whose presence drains hope and happiness and who can cause severe psychological suffering.
The prison is first discussed in Harry Potter and the Philosopher’s Stone and becomes central to Harry Potter and the Prisoner of Azkaban. Sirius Black’s escape from Azkaban drives much of that novel’s mystery, because the wizarding world believes he is a dangerous murderer.
Azkaban is often associated with the Ministry of Magic, but its history predates the modern prison system. The fortress was originally built by the dark wizard Ekrizdis. Rowling’s later material explains that the site’s isolation and magical defenses helped conceal its existence from the non-magical world.