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.
First off I’d scrap the CO explanation, but keep the theme of it. Call it Crawfisher’s Odor, CO for short. This gas forms no more than a meter below the surface, keen eyes can spot it gathering by the rising of the swamp’s surface. For many this is too late as soon it bursts spreading its [select colour] miasma. It is heavier than air gathering in low areas and slowly dissipates.
So now we have something that
a) can be detected (the rising of the ground, like a bubble about to pop)
b) can “easily” be avoided by finding higher ground or climbing a tree
c) offer a risk-reward situation (can I get to the next higher ground before asphyxiating as [threat] is onto me)
or
c_2) do we go straight through it (high risk) or take the long way around it ([threat] gets a free advance on their thing)
Also the who swamp is difficult, borderline impassable, terrain. Locals are used to the dangerous CO using stilts for travel (difficult check for characters unused to them) and every dwelling is at least 2m off the ground.
That is just the danger of the CO. Add in sinkholes, terrain that looks firm but really isn’t, big ass snakes and cannibals and your players will never want to delve into it again. But then the cities of [old wealthy empire] are there ripe for looting…
Oh, I like this much better than mine. My brain got stuck on the specifics of using a real gas.
In hindsight I prefer your suggestion of a visible miasma, may also make it flammable because who doesn’t like big booms? Bubbles also define a nice area which an encounter could happen in - a radius area that fills up with the gas which the players could then escape from.
The locals here would mostly be rampaging undead that don’t need air, though even they might not want to get stuck in the mud here.