In my fantasy world, there's something called the Darkness. It isn't real in a physical sense. It's more like a living idea. It embodies all of our dark emotions—fear, anger, hatred.
It is drawn to and fueled by these emotions, and it has a stronger presence wherever these emotions can be found or where the emotions are associated with. If you've got a lot of really angry people in one place, or a single place that everyone's afraid of, the Darkness will have a strong presence there.
How could you remove the Darkness after it comes to dominate a place? Simply fighting it only fuels it, so you have to find other ways. The main people who would be equipped to handle the Darkness would be mages, whose magic I discussed in part here. I'm trying to think of something between psychological manipulation and some sort of magical ritual to banish the Darkness from an area.