I know "every dungeon has a difference"...that's what everyone says about their dungeons.
But here's one I'm working on.
The basic premise...
Generations ago, something hot fell out of the sky, burning a huge shaft into the ground.
Halfway down the hole it left, there are thick oily mists. Sometimes the sun shines at the right angle to burn away some of the mist. On those days there is a structure visible at the bottom of the hole. The structure has great cylindrical chimneys that burn with red heat, constantly churning off more of the noxious fumes and mists that fill the middle layers of the hole.
Trying to climb down the central shaft is nearly impossible. The stone walls have long ago been melted into a hard glassy obsidian, and those who do make the climb are often picked off by monstrous bird-like creatures that perch in the few cracks.
Riddled around the main shaft are thousands of caverns and tunnels, a three dimensional labyrinth through which strange creatures ascend from the depths. Adventurers claim that there are magical artefacts and riches beyond imagining in the "crooked castle" at the bottom of the shaft, there is no end to the number of brave or foolish souls who try to find their way downward. Fewer are the ones who emerge.