You managed to trap the remnants of a smell-o-the-wisp in a glass bottle -- you keep a supply of them on hand, in case you want to collect some troll sweat or whatever. The empty ones don't show up in your inventory screen, because why bother? They're empty.

Anyway, you figure you could maybe throw this at someone, and the bottle would break and they'd smell like gross swamp gas, which would be unpleasant for them.