Edit: hold on… THE Nepenthe? Who likes yarn and cats? What a nice surprise running into you here

I KNEW IT WAS YOU. I was just debating saying something in the discord! Both a pleasant and hilarious surprise

I'm thinking for what they want, something inert would be a good deal, yeah. Just nitrogen can be made in quantity pretty easily, would displace the air enough to put out a fire, and cause disorientation, possibly nausea. Main problem is, we would be talking brain death in a little over a minute and that may not be enough time if the party is dnd stupid. A TPK in the span of a couple breaths, because without the CO2 you wouldn't understand what's going on until someone noticed the torches, and they may be too out of it by the time that they do.

Kinda wanna go through a list, now, to see if there's anything heavier than air that's entirely or quickly becomes odorless, has more pronounced symptoms than "confusion," and wouldn't kill quite so fast, but I'm not entirely convinced because of....how necessary breathing is. The rotten egg smell of hydrogen sulfide dissipates very quickly into nothing, but one breath is enough to kill you and that isn't hyperbole.

All that would, of course, be assuming we're playing by irl rules despite the whole place being built by a zombie wizard.