|Found on Wikipedia|
Fog is especially wonderful in RPGs because of the inherit danger of itself. Getting lost, things hidden in its tendrils, lack of vision and losing companions all are very real terrors inside of a fog. Using a poisonous fog like chlorine gas amps the danger up.
Setting up a fog encounter for overland travel is straightforward. Just have the fog roll in on the characters. This alone brings up the tension level, especially in players that have dealt with Ravenloft in the past. But, this does not make the encounter a "trap." It needs additional elements. The fog is just a clue that a trap is imminent.
The next element for using the fog as a trap is a change in the environment. A wolf howls in the distance, with a response from a different direction would be a good example. Now, the players know that something is different. The "landscape" has changed. 5E players should be using Nature or Survival rolls at this point to figure out how to get away from the wolves. OSR players are probably asking for exact descriptions of trees and surrounding environs. (Don't give it to them. They are in a fog, remember?)
At this point, they need to run away from our trap, or spring the trap. Either way, the action gets intense. Running may lead to separation or getting lost. (5E DMs need to use penalties to Survival/Perception/Nature to mimic the fog's effects on the senses.)
The fog in the previous example acts like the tripwire or pressure plate in a typical trap. It lets the players know something isn't right, and something bad could happen. The howling of wolves pushes the characters to action (and actually springs the trap.)
|Image from Skyrim|
Hitting a pressure plate (one of our classic trigger elements) the party causes the fog to roll into the room they are in. We could use a Save or Die for chlorine gas (in OSR). A more interesting variant though is concealing something valuable. The trap door in the ceiling actually is a hidden way out of the dungeon. The players will be too worried about surviving to find the hidden prize, though. After 5 minutes the trap closes and resets. Whatever the fog hides, have the players make Saves (PPD for OSR, Con for 5e). Nothing bad happens when they fail, but it keeps them guessing.
Until next time, see you in the dungeon halls.