Taking inspiration from the Kickstarter-funded movie
Dragon Mountain (trailer:
https://www.youtube.com/watch?v=LsG3XShs7JE).
If this is a one-off, use pregens with all characters being humans or dwarves. But you could also use their existing characters, come up with some reason they are in the mine (perhaps they were hired to root out who has been sabotaging mining operations).
The session starts after a mine collapse. If pregens, the party is trapped together and need to find a way to work together and escape. Otherwise, the party finds itself trapped with a small group of dwarves and humans.
The mine is owned by a powerful human empire that employs Dwarves to operate it. Due to the empires hegemony, territorial disputes, and insatiable need for material, there have been occasional hostilities between it and many of the dwarven clans. The dwarven minor harbor enmity towards and mistrust of humans.
One of the dwarves (either NPC or a player if using pregens) is a member of a rebel group deemed a terrorist organization by the human empire. S/he is responsible for the collapse. It was not intended that anyone would die. The intent was only to disrupt operations. But s/he did not know about a number of secret passages and rooms that the humans had created, which weakened the structural integrity of the mine. The humans took measures to make the mine easily collapsible should the dwarves turn against them. This dwarf will be unfamiliar and suspicious to at least one of the other dwarves as s/he is not from their clan and new to the mine. This dwarf is also the only one who knows how to run the mining machines (see below).
The collapse opened the mine to part of the underdark and to the lair of a subterranean beast that feeds off of metallic minerals. The party and NPCs should see it as an existential threat that causes them to work together. You could reskin an ankheg or rust monster (or several). There should be non-combat options if the players want to try that. In the movie, the creature was not evil, just a beast, and it was after their oil not to eat them. They were able to distract it with "food" and also make away with some of its eggs giving the minors much needed food. This was after combat weakened it. So you can have a bit of both or just play it as a combat threat.
One of the humans (NPC or pregen player) knows of some secret passages that can provide shelter, make escape easier, etc.
In the movie the steam-punk them added much to the movies flavor. It created a resource challenge and getting them to work and keep working were important for light and for possible escape. For D&D you can make these machines or magic devices. Light might not be so important if you are playing with a lot of dwarves, but if many of the party do not have dark sense, the fear of having no light source can make it important to keep the machines fueled and running. I would say that you have challenges to: (1) keep light sources fueled, (2) run machines that bring in fresh air and remove dangerous gases (if the party doesn't get these running they will eventually suffer levels of exhaustion until they die), (3) communicate with the outside world (which will give them invaluable tips to succeed).
Getting the parts and fuel for the machines should involve clearing rubble, skill challenges, avoiding the dangerous creatures in the mine with them. Some parts should be locked in secure storage rooms and safes that require puzzles to solve, locks to pick, etc.
If you want something more fun house with more in-your-face puzzle challenges, the collapse instead reveals an ancient dwarven tomb, which may provide a route back to the surface, but only if they solve the puzzles, avoid the traps, and defeat the undead challenge. But I like the flavor of the escape the mine by putting together machines by avoiding strange beast. Any puzzles I would try to work into putting the machines together. Perhaps the machine to communicate with the outside world was meant for high-level management only and no one who can operate it survived. To operate it you must solve a puzzle meant to keep the average minor from ever using it.
