Cloudkill

The cloudkill spell generates a billowing cloud of ghastly yellowish green vapors that can be deadly, either instantly to lesser creatures, or after extended exposure to more powerful ones. The cloudkill moves away from the spellcaster slowly, rolling along the surface of the ground. A moderate breeze causes it to alter course, but it does not move back toward its caster. A strong wind breaks it up in a few minutes, and a greater wind force prevents the use of the spell. As the vapors are heavier than air, they sink to the lowest level of the land, even pouring down den or sinkhole openings. It cannot penetrate liquids, nor can it be cast under water.

Note: This version of cloudkill originally appeared in the 2nd Edition Player's Handbook for Advanced Dungeons & Dragons from TSR. Its use here is for the purposes of providing context for the campaign only.