Azkaban Prison
From Hayashi Park Potterverse for 5th ed. Dungeons and Dragons (5e)
Prison fortress where wizarding criminals are sent. Guarded day and night by the terrifying Dementors, who drain peace, hope and happiness out of any human in their vicinity.
Located somewhere in the North Sea, it has been rendered Unplottable (unreachable/unmappable) by a Masking spell.