The bigger question would be.. Why?
This may sound cool. But in reality it will be extremely boring having to backtrack to a safe area, press R a couple of times and then continue your journey.
Monsters that are not poison immune will die by the poison. There are so many "if" and "what if" situations.
Why don't you consider other more practical solutions, like entire sections cut off by the black mist. You can move around in black mist areas getting poisoned just a few squares to place a torch in a torchholder. invisible blockers keep you from advancing into the specific area with a warning that the black mist is too strong there to advance.
This way you don't have to spawn the black mist everywhere and only on a few squares just for looks.
and the torches are some kind of currency that can be placed into torchholders to clear a big area or even an entire map.
This way you only have to place a simple poison mist script hooked to a timer with invisible floor trigger when you enter the map and disable the script when you leave the map.
As soon you place a torch in a torchholder, the script is stopped and the floor triggers disabled.
The blockers are removed and you can explore the section of the map as soon the last poison cloud has dissipated.
Use the bell sound as a cool animation/cutscene/scripted event when you're already in a safehaven, black mist is activated everywhere around the safehaven gradually spreading comming out of floor grates and wall gratings or something like that. Invisible blockers are activated to keep the party trapped in the safehaven. And you have to do some kind of puzzle to get to the torches that have to be placed strategically to open up the way again.
This is how I would do it.
Scripted events where you have something to do is much more enjoyable to the player than some random obnoxious backtrack mechanic that's completely negated by pressing R.
As for ideas to replace the default looking poison cloud, I would look more into the direction of the graveyard fog or something alike. And I have some demon face that moves towards you in a fog square as a scare effect, but i can't remember what it's called. Or from which asset pack it came from. I think it's from Thom, but I'm not sure.
Also check out the effect of a single swamp_dungeon_fog object on your map and see if you can combine the object with a "fade in" script or if you can make it into a custom object with different colors.
https://youtu.be/jf-UjwmTKWc