With named poison gases one would find irl, there really isn't a lot available that would meet the requirements. 99.9% are too volatile either to open flame or the presence of oxygen, have a very strong smell and/or obvious color that one probably couldn't explain away by blaming it on the tinted glass of a nearby lamp.

Or they cause immediate symptoms that the party would be far too damaged by to doubt what's happening (blindness, lung damage requiring immediate medical attention, etc.), or the symptoms don't show up for 3+ days or require months of exposure. At which point, I consider any lich really wouldn't bother doing that on purpose because who's it going to slow down? It would be more of an accidental environmental feature like radon that would leave the surviving players lingeringly convalescent just to piss them off.

For something so non-reactive it poses no danger to the big bad, with immediate symptoms that are still deadly while being understated enough that an adventurer might brush them off...it would more or less have to be mostly or entirely inert, I feel like. Nitrogen, argon, and krypton are all heavier than air, inflammable, unnoticeable, and all easily made through an identical process at differing temperatures, with krypton additionally exhibiting a strong anesthetic effect.

A study of nitrogen gas as a replacement in fire extinguishers doused varying types of fires in 11-71 seconds. So your torch would go out, yes, which would be really cool. With this one, at least, it also seems to have a habit of reigniting when you leave. Fun!

However, my main concern with this would be that because all you're doing then is displacing the air with something your body doesn't even notice. There will be progressive symptoms as one approaches the source: fatigue, a feeling either of panic or drunken euphoria, dimmed vision, dizziness/incoordination, nausea and vomiting, confusion and hallucinations that may make them roll at a disadvantage to address any of these things.

And then you fall over and die. These things kick in very fast irl and kill even faster. As in, unconscious in half a minute and brain-dead in ten minutes or less. This would obviously be unfair to the party. Your light would go and then you go.

The two thresholds are very close for my liking, making the question in my eyes how fast they would notice anything is wrong before they had to grab a new character sheet. You'd have to fudge reality a bit if you wanted to make this even a little bit not bullshit.

And in a setting where the lich has intentionally depleted an area entirely of oxygen? We already have places like that. Pluto's Gate in Turkey emits a steady stream of CO2 that pools on the floor of the cave in such strong concentrations that anything passing the threshold at the wrong time of day is dead unless it can hold its breath. You didn't get a warning beyond what the locals would tell you and probably all the corpses. In roman times, they sold birds for the tourists to toss in. You're going to have to make something up.

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.

Ok, now what's the result for CO 1,000,000ppm?

That's what the Lich has done. It has filled the space with CO.