The Demon's Veins (2019)

From publisher blurb:

A band of dwarves have hacked their way into the side of a mountain, hoping to strike mithral and make their fortune. So far, they haven't found any — but they did uncover a large hollow inside the mountain and a demon lurking at the bottom of a crevasse. To make a little money until they strike it big, they rope off the crevasse and set it up as a tourist attraction: See the demon in the Pit of Hell! Can a mining colony of dwarves the a spawn of the Abyss living next door coexist peacefully side-by-side? Some outcomes are more likely than others — and the most likely outcome is a mess that needs a bold party of heroes to clean it up.

The Demon's Veins is published under the Open Game License. It does not specify a particular campaign setting, and it allows you to integrate it into your own campaign world by filling in various details. You can use it as a stand-alone module, or as a sequel to Places by the Way #5: The Dwarven Smith's Tome.