The weather in the region has always been a bit unpredictable. One thing is certain though: when the clouds dip low enough, they bring strange horrors with them. Write about a newcomer to the region receiving a warning about the impending weather shift, and what they experience when the mist rolls in.
Ideas and questions:
If the newcomer did not arrive on their own, you can turn the story into a frightening tale where the horrors affect the characters one by one.
Perhaps the newcomer is already aware of the strange horrors, and that is what brings him to the region.
Does the newcomer get a warning well in advance of the phenomenon, or do they get the warning as the horrors are about to unfold?
Are the horrors that the clouds bring alive, or metaphysical?
Do the clouds dip low enough that even those who are in the low lands can be affected? Or, can people race for safety to lower ground?
Explore the story from another perspective. Perhaps the newcomer has always lived underground, or they lived in a heavily forested area. What if they had never seen trees before?