Vapor rats appear to be nothing more than large, gray, giant rats. Their habitat, however, includes areas not common to giant rats, for these creatures also dwell in and on the substantial cloud islands that frequently serve as the abode of cloud giants and cloud dragons.
If angry, hungry, or cornered, vapor rats attack by scurrying in and delivering a sharp bite. Whenever one is killed, it gives off a small puff of nauseating, noxious fumes. The rat always directs its release toward its opponent, and the gas dissipates to harmlessness beyond a short distance. Thus, while it is safe to slay these creatures at a distance, they are particularly dangerous in close proximity.
It is possible for vapor rats to alter the substance of their bodies and assume a gaseous form. In this condition they appear to be wisps of cloud or similar vapors. In their vaporous condition they are able to direct their movements much as a ship would steer before the wind, and they are thus able to move from cloud to cloud around the sky.
Wounded or seriously threatened vapor rats always assume gaseous form. In such a state they can be harmed only by attack forms that cause their vapors to be destroyed, such as magical fire, lightning, and exceptionally strong winds.
Note: Vapor rats originally appeared in this form in the 2nd Edition Monstrous Manual for the AD&D game from TSR. Their use here is for the purposes of providing context for the campaign only.