It written that there is a doorway, a portal between this world and the next, which stands closed.
It is rumored that if this doorway is ever opened, then the Deep Old Ones will return, and plunge all the world into utter darkness and chaos.
It is suspected that this may have already happened.
Once every 253 years, the energies of the universe align in just such a manner as to allow the opening of a gateway to another dimension. When this occurs, two groups will gather: Those who wish to open the door, and those who strive to keep it closed. The only problem is that no one knows who's on which side.
This scenario was inspired by Roger Zelazny's A Night in the Lonesome October. I highly recommend it.