Thanks for the pointers on the gas types there, had not thought of that as much - more just what I wanted it to do.

Yeah it's DnD5e.

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.