Dense, swirling fog is a classic component in adventure writing. Things—terrible, unseen things—lurk in the fog. Danger lurks in the fog. Sometimes, the fog itself is the danger. And sometimes, the fog has other strange, magical properties.
The characters are deep in the wilderness. They come across a deep valley filled with billowing fog. What lies in the fog? What lies beyond the fog? Dare they find out?
Quiet! Did you hear that?
Remember, for the writer, designer or GM inspiration is everywhere.