This reverse of cloud of purification creates a billowy cloud of vile vapors that moves with the prevailing wind at a rate of 20 feet per round. A strong wind (more than 18 mph) breaks it up in four rounds, and a greater wind (a gust of wind spell, for example) disperses it in one round, as will thick vegetation.
The cloud transmutes pure water into organic filth and rotting garbage. Small animals (mice. squirrels, etc.) mutate into rats and rot grubs. Flies and other foul insects immediately rise from the mess. If cast over a sewer or garbage heap, the spell doubles the filth created. The cloud itself lasts for only one round per level, but the effects are nonmagical and permanent until magically purified or negated by time (though flowing water also tends to eliminate the effects relatively quickly). This spell can spoil a castle's water supply or foul a small river. Depending upon what the caster uses this spell for, it may well require a Ravenloft powers check.
Notes: Common on the Demiplane of Dread; used mostly by evil priests.