I'm running a wilderness exploration campaign currently with lots of hexcrawl sections, and my players are about to progress to a new zone.

One of the hazards I've decided should be here is a low lying swamp of decayed plant life that has an layer of asphyxiating gas (a big layer of carbon dioxide sitting on the ground) over the area. Idea is it's a dangerous biome with a non-obvious hazard unlike ones they've seen before in the more traditional forest and mountain settings.

I am strugging to come up with a way to make this idea into an encounter that would be satisfying for the players, not wanting them to all end up asphyxiating in some dire swamp after failing a roll or 2 but still having a level of risk and the option to use their abilities and such to overcome it.

My current working concept is this: -On attempting to enter this tile, perform a Constitution (Perception) check against DC 15. On a success, you notice that the air here is making you feel light headed and rapidly ve