Otherwordly Prison (Patreon)

This mysterious prison is set in a demiplane out of reach of most. What kind of creatures are held captive in its dark cells ? Who runs the place ? What are the effects of this strange mist coming up from the nether depths ? Maybe your PCs are imprisoned here and need to escape. Maybe they need to rescue a powerful NPC or need to interrogate a powerful evil creatures kept here ? Up to you to imagine the purpose of this location.