Long time ago all these mountains belonged to dwarves, then came humans, elves and other folks from the North. The situation was not very easy at the beginning but finally they managed to settle it. All the lands of the nearby kingdoms were united into an Empire. The kingdoms still exist and have their own rulers but all of them are under the emperor. Dwarves agreed to “share” their mountains with others, so the Western part of the large mountain range was cut off their kingdom and given to the neighbor. There were several dwarf mines there. Some of them were taken into work immediately, others forgotten. But now the kingdom and the whole empire are growing and more resources are needed.
So, a local lord (action takes place in the mountain region) offers PCs a good job. If they manage to get into the mines, explore them and draw a comprehensible map they will get a nice reward from the King. If they also find all the traps (either disarm them or indicate on the map), possible hidden locations and possible unpleasant surprises left by dwarves and also kill possible monsters - they will get a double reward.
To get the reward the PCs will have to wait for a week or two when the first group of miners get down into the mines and check all the drawn locations.
In fact there are a lot of unplesant surprises in the mines. Traps, hidden mining locations (with especially valuable minerals), monsters and natural hazards.
Under the mountain there also lives a tribe of some small and quite harmless creatures like kobolds. Their own tunnels led to one of the lower caves of the dungeon. There they might meet the PCs and try to talk.
If the PCS are clever enough not to start a fight these creatures can show them a save pass to some secret location. They are extremely afraid of going there otherwise they would have blocked the passage long ago as they feel some strange vibration going from there. They believe the place is cursed. The passage to that cavern is now full of poisonous gas and anyone trying to go this way will die for sure. But the kobolds have their own safe ways and secret passages. If the PCs use them they will find an artificial passage and a cave of about a thousand year old. In a large hall there is a strange artifact, a mechanism of unknown function which makes those vibrations.
There PCs also meet spirits of some ancient adventurers who tell their sad story. They came here about 600 years ago looking for treasures of an ancient civilization and found this mechanism. They switched it on out of curiosity and suddenly were attacked by a horde some demonic creatures. They were killed and their souls trapped by this artifact. They hope if it is stopped the curse will end.
The mechanism seems to be incredibly difficult to understand but there is only one lever. The spirits say that they pulled it down and all this happened. Then the lever returned to its middle position. Now it may be put either down or upwards.
The clue to the mystery is drawn on one of the walls of the hall. If PCs make a successful Search check under the dust and cobweb they will find a drawing (looking like Egyptian - everything from a side view): there are two groups of humanlike figures with a symbol of the Moon above them and the mechanism sketchy drawn.
The first: the Moon is in growing phase, the lever is in the lower position. Rays of light are descending upon the figures, their hands are lifted up.
The second: the Moon is in decreasing phase, the lever is in its upper position. No rays, the figures are turned away from the mechanism as if leaving.
If asked, the spirits would remember that they came here shortly after the full moon.
The solution of the mystery is in the Moon phase of course. The mechanism was switched on (lever down) in growing phase and switched off (lever up) in decreasing one. This constant process of switching gave eternal life to the creators. A special group of priests was to look after it. If the lever was pushed the wrong way a horde of demons was summoned to kill them and then the mechanism trapped their souls taking their energy. As the creators of the mechanism are absent for many centuries these souls were not absorbed but only kept. Now everything can be finished by pulling down the lever in the decreasing phase of the moon.
As I gave my PCs a calendar with dates and Moon phases at the beginning of the campaign, it won’t be difficult for them to remember or count up that the decreasing phase will begin in two-three days from now (depending on how long they have already spent underground).
In fact, if they do something wrong no demons will be summoned as the powers of the artifact are extremely low now (no one was attending to it for about a thousand years). But the trapped souls will be taken into the Abys, a terrible plane of torture for ever.
In the end there is a choice for PCs: to report about the artifact to the authorities (so that its powers may be learnt and used again) or ask the kobolds to block all the passages leading there forever. Another choice concerns kobolds: to help them sign a treaty with the local authorities or to announce them dangerous and convince everyone of destroying the tribe.
In any case the full amount of experience points will be given to the PCs only if they present a nice scheme of the whole dungeon to the Master at the end of the module.