Tell your players you are running a sort of incursion campaign. Paladin oath of the watchers sort of thing. Play a class or character that has that sort of purpose. They just can't play a dwarf, halfling, or gnome (no small races).
The party meets at the foot of the mountain Hall of the dwarven king Faremgaull. Long abandoned, his kingdom remains largely unplundered and likely will remain so due to the rumors of the monsters that creep in its depths. It is these monsters which have brought your players here. The duchy at the foot of the mountain has a rotating guard post at the entrance to the dwarf kingdom. They know there are monsters within, and those monsters will creep out if no one stops them. Even in the depths of Faremgaull's hall and lower into the mines, these monsters wait till the surface is dark to make their move. The party is encouraged to enter at least the central promenade of the city to see what sorts of things to expect. The city is largely in tact. Great works of metal and stone still stand. Statues, tunnels, and arcane works remain with not a dwarf in sight. It's eerie.
At night, they return to the guard house where the duchy's wizard has left some equipment. A large arcane cube whirrs to life at the main desk of the outpost. A voice, warbled and scratchy, as though recorded in a hurry speaks through the magical device. "Hello hello!..."
The cube gives instructions. The guard squad of players must remain at the outpost and keep watch on the defunct city's main gate. The gate must be left open in order to let air out and prevent the buildup of toxic gases due to failures in outlying parts of the ventilation grid. It can be closed only for short periods of time or else they risk explosions that will rock the whole mountain. The autonomous clockwork guard of the city will potentially still patrol as well, but they shouldn't leave the confines of the inner city. A scrying orb linked to the city's arcane power grid has been left in the outpost so that watch members can monitor various parts of the city and see potential threats coming.
The players must prevent monsters from escaping each night until their shift ends. It is a lucrative post, but many rumors say few return. But it's only a five day shift.
What could go wrong over the course of Five Nights at Faremgaull's?
