Assuming the game supports creating temporary regions of non-poisonous gas clouds: New “smoke bomb” potion. It makes you choke/vomit when quaffed. Intended to be thrown; where it shatters, most of the surrounding terrain will be transformed into cloud.

Vanilla has since added non-poisonous gas clouds (for things such as mist from zapping fire over water), and this effect could work just as well by breaking a smoky potion, instead of adding a new object.