I made this diorama/terrain piece for my D&D session last Sunday. It turned out quite nice, I think! The mist is generated by an "ultrasonic atomizer" I got on amazon a while back, with the piezo wedged into a chunk of foam insulation, so it floats on top of the water. The effect was slightly undermined by a continuous not-quite-ultrasonic whine from the piezo in person. Luckily, zoom filtered out that higher frequency, according to the players joining online!

Click here for mild Call of The Netherdeep spoilers

The scene is the entryway to Betrayers Rise, in Bazzoxan (Chapter 3):

You stand on a bloodstained bridge that extends over a chasm of swirling mist. On the far side of the bridge a is a closed double door of black stone inscribed with a circle containing the profile of a smiling woman. The symbol is gouged with deep marks, as if something with claws has tried, but failed, to scour it from the stone

This is where we started the session from, and although no combat takes place on this bridge, I think it went a good way towards setting up the mood of the betrayers rise: dark, mysterious and reasonably dangerous. The actual interiors of the dungeon I ended up drawing on some paper with a sharpie as we went along.