This map has a story built into it, but more from an overall backstory for the whole thing, rather than a room-by-room breakdown. So this isn’t going to become an Un-Furnished Dungeon (although it would be easy enough to key it up and lay it out in a booklet form).
This was an earlier version of the same sort of feature entrance I used for my bit of the Exquisite Corpse dungeon.
It started off with the opening maw (which I obviously thought was a sufficiently cool feature to plagiarize myself later on), but then I got distracted by the idea of contrasting the very twisty, writhing passages with some very square rooms. The four corner blocks were set, and it seemed interesting to have those connected to the rest of the map with straight line halls. The intersections between the two then became a kind of cuff-like knuckle, and the image arose of this as a set of confinements to restrain a huge monster.
It reads as some kind of wyrm or tentacled beast with a great open mouth that had been shackled and held in some fashion, but now vanished. The tentacles, and the blocks and shackles that had anchored it, have all disappeared, and the open passages are now what remains in their place; it is a negative space created in rock from what was once there.
Was the creature buried in the ground and then, using its powerful but unknown magical abilities, displaced itself into some other dimension, leaving the space where it had been carved into the side of a mountain?
The outline for this was done a while ago, and I only recently got to adding some hatch to it. It seemed to call for something different than the usual ground hatch, and this seems to be a good alternative for this case.
As usual, feel free to use for any non-commercial purpose, or contact me if you’d like to use this on a commercial project.