So, I started a thread based on someone's complaint that DF is too easy. And people've been talking about implementing goblins that can dig. I thought that was interesting, but might break the game for some people, and it really doesn't go far enough, since it's still pretty simple to deal with that problem...
So I came up with something a little more extreme...
Earth demons, roughly in line with the earth elementals from D&D, etc. Demons that can travel through stone, either by digging really fast* or via some kind of stone osmosis. Being earth-based, being cast in obsidian doesn't kill them, though a cave-in still might. And being earth-based, they must unable to fly, because that would be too much.
Since such monsters would be absurdly overpowered and clearly break reality, I'd rather not have them appearing outside of the HFS. I also would not recommend making all of the HFS into power-diggers, but may make it one of the random qualities a demon can possess such that every HFS breach will generate a few of these nightmares, just to make hell a bit more !!FUN!!
I'm not sure if this idea is a joke or not.
*Anyone else her remember how monsters could dig in the old game T.O.M.E.? Kind of like that...