So, a long time ago, I read an article in some nature thing or another. The article suggested that bees build their hives differently based on past events; like a more complicated version of tree rings. "Ah!" said the young bee "I see by the fact that the hive entrances are small and east-facing that we're at war with the neighboring hive! Good to know!" Regardless of how accurate this is in the real world, I connected it to dwarves. What if they had a (literal) hive mind?
Entire mountains given sentience by cavern halls covered in elaborate runes; or maybe just as a result of belief. Maybe something inherent to the labyrinth tunnels of the dwarves. consciousnesses that span millenniums acting as guiding forces for whole civilizations. Dwarven high priests learning to commune directly with The Stone. Maybe the really big, really old ones get to be demigods in their own right. Maybe there's a campaign finale where someone tries to bring the whole mountain to life, so it can scuttle around grazing on entire cities?
But then I had a thought. What would a map of a mountainholm look like? Well, your stereotypical mountainhome is going to have one massive cavern hall, and then maybe lots of little tunnels that branch off from it. Of course, you'd want a 3d map to be most useful. Maybe one you can carry around?
So then. There's some dwarves who are interested in retrieving some stuff from a mountainholm that got lost beneath the sea strange aeons ago. Archeology, pretty straightforward, not terrifying stuff. Maybe they hire the player characters, maybe they just keep cropping up on the sidelines. Then it turns out their plan is a bit more ambitious: They want to raise the whole thing and create a new island. The players don't see a problem with it. Then there's the reveal: The forgotten city probably should've stayed forgotten. Maybe the surface dwellers legitimately thought that they were just going to find priceless relics or happy-go-lucky long lost cousins, or maybe it's a cult trying to end the world. The important thing is that the dwarves that stayed below have adapted to their new environment, and coincidentally gone insane.
Now the race is on to prevent the Great Old One from returning to the world! It's time to go
In The Hall of Cthulhu the Mountain-KingSo anyway, that's the basic layout of the thing. I'll keep updating this post as I flesh it out a bit more and actually begin to write up primary NPCs and specific objectives and artifacts