It’s a potion that absorbs gasses. “Well that goes right on the old post-adventure sell pile” you think. But it’s actually pretty cool.
You uncork it and it soaks up any gas in the area. If there’s natural explosive, suffocating, hallucenogenic, or poison gas around you’re safe for a while. It also sucks up any gas spells like Stinking Cloud, Cloudkill, Incendiary Cloud, or Green Dragon breath. Up to your DM whether it sucks up Fog (I’d say sure) or a gaseous Vampire (which would be hilarious).
The best part is when it’s soaked up the gas, you can cork it and later throw it to shatter and release the stored gas. This way you could grab some nasty gas you find in a dungeon that’s supposed to be a trap and use it later as a weapon!
Let’s say you find 3 vials of this stuff. You might think it’s a great idea to just load up all three and have 3 weapons available. But you’ll probably find interesting gas in the dungeon, so save a couple to use in there.
By the way, normally I wouldn’t bring up any sciency mumbo-jumbo, but if you drink this stuff your blood can’t carry any air. You suffocate using whatever suffocation rules exist in your game, and Water Breathing won’t help. You’d need a Dispel Magic or something that lets you survive in a vaccuum.