In the Discworld series by Terry Pratchett they have a legend about the Knockermen told by the dwarven miners. The general upshot of the concept is that dwarves that have died in cave-ins deep in the earth are trapped there, body and soul, and become the knockermen. When a dwarf is working by himself deep in the mine, sometimes he can hear the sound of someone gently knocking on the stone around him with the tip of a pick, as the knockermen search for a way out of the depths of the earth.
Perhaps this could be the basis for a “monster” and location to be used in the mining depths. Perhaps a small storage vault long ago buried and forgotten until discovered by the hearthlings. A small cache of ores guarded by ghostly hearthlings that attack on sight and make a beeline for the surface to take revenge on those that never came to rescue them.
Implementation would be simple enough. Existing character models set to semi-transparent and randomly generating their room well underground. Then just set them to attack on sight, path them toward the town banner or stockpiles, and off they go the instant that room is discovered.
If nothing else, the knockermen could make mining a bit more exciting while adding a bit of risk for reward to digging a hole.