The skeletons could mine 24 hours a day, 7 days a week. Nonstop. No need for shifts.
The evil entity could be a lich or other powerful undead, trapped in a sarcophagus buried in the rock. A totally sealed rock would prevent a vampire from turning to gas and escaping.
Yep, but that wouldn't be imitating their former lives, which all these skeletons do, on auto-pilot. They behave like the miners they were.
Lich? I'd have to seriously weaken it somehow. Problem is, a monster that makes sense will blow the party out of the water. So I'm leaning towards artifact.
Then why do they need deathloks and wights supervising them?
Do they go back to their homes and pretend to eat food and go to sleep, and all of the other trappings of their lives?
Just curious. It just seems that if the skeletal miners are acting like in life, that it would go beyond mining and go into things like "going to the local market to buy food", etc. An entire village of skeletal workers would pretend to be actually living.
It could be a revenant or some lesser form of undead. Possibly an undead cleric whose god still gives him the power to animate other undead.
Artifact is good too.
The "Thieves Guild" series of books had deities that were super weak because they no longer had worshipers. The "Chosen" (i.e. right hand man) of such a being could be in a coffin, buried in the rock. His goal. Bring his master back into the pantheon of more powerful deities. But first, he has to get free. And he himself is super weak (at least compared to what he was millennium ago).
These would all probably involve a bit of a twist on your original idea, but you might be able to use something from them.
This first idea would be about on par as a challenge for your group and would be an open and shut plot.
Faith Runs Deep
A buried monolith that is host to the essence of an evil or elder god. Perhaps:
* the god itself is directing the undead in an attempt to free itself,
* a servant of the god is directing the undead to free the god,
* a servant of the god is directing the undead to free the god, but it's just a legend and nothing (or something else) is down there.
Success: the monolith remains buried, and the powers of the god remain dimmed. The PCs learn a secret that will need to be guarded.
Failure: the monolith is released and the power of the god will begin to grow. The PCs can't destroy the monolith yet, so they must flee and set about imprisoning the god again or find a means of destroying it.
Where'd you learn the technique to frame scenarios with success and failure? I love it!
First off, I really dig the word "monolith" and love the visuals, but it doesn't sound portable! I'm leaning strongly towards an artifact that holds essence of an evil, forgotten god. The problem with the failure scenario is it's like jumping into a PhD without thinking it through -- I'll be with this campaign story for years, I'd better be sure I like it!
Help!
Imagine that the mine in Wave Echo Cave is up and running with skeletons mining away as they did in life. Every day three teams of 10 skeletons gear up with picks, hammers, chisels and shovels and put in an 8-hour shift under the supervision of some deathlocks and wights.
There's more to it but here's the thing: the undead are digging to free a dark force trapped in the rock, encased there after losing an epic battle long ago. It's no coincidence that the power is near the mine:
[snip]
I need to think outta the box. Please sprinkle some imaginative sugar.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.