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 very out of breath. On a fail you go deeper in and everyone starts suffocating. Either way, all torches currently being carried go out. If they go out, everyone makes DC 10 CON saves to escape without issue On a fail, they start suffocating and the encounter begins 100 feet from the edge. On pushing deeper in, everyone makes CON saves and the worst person (or persons if they tie) begin suffocating first, with everyone else starting to suffocate a number of rounds later according to how good their save was. Encounter begins 200 feet from the edge. When suffocating you have your CON mod rounds until you drop to 0 hp. The encounter here would be an initiative roll and then they could proceed to escape and rescue their allies - no monsters.