I don't know of one, but you could have it so that the Zhents are tossing in prisoners into a demi-plane and it's close enough to the astral plane that it attracts the attention of the Astral Dreadnought.
The old 2e product "Country Sites" has a pretty cool prison complex set in the midst of a large swamp. Or you might try the old Dungeon magazine adventure that has a giant wheel prison carved into a mountain (I can't for the life of me remember the title, though).