This is what will happen if the heroes don't stop the Big Bad or the Omnicidal Maniac from doing its nasty work. It can be either supernatural or superscience, depending on the villain, but in either case the bad guy must be beaten down and his toys broken in order to save the planet, or the universe, depending on the focus of the story. Usually it's figurative -- expressed as "merely" the death of humankind, the obliteration of Civilization, or its subjugation to aliens, for example -- rather than the literal rendering of the planet down to gravel.