Oh no, what if it's a zombie army, gaining in size by taking corpses from their graves and taking them down to their underground castle where they infect the corpses with the zombification disease that re-animates them?! They could have been down there growing more powerful for thousands of years, and now, they're rising to the surface to infect all humans! Then, zombies will become the dominant species on the planet! We have to stop this before it gets out of hand. We could go to the army for help, in search of a nuclear bomb, only to discover that the disease has already spread there! Then, we'll make it to the bomb storage room and activate the bomb, then escape before the bomb explodes, only to come to the realization that the explosion is too massive and that it destroys the entire world, including us!