Green Rain (Evil Weather Supplement)
From D&D Wiki
 What is Green Rain?
Green Rain is a thoroughly evil phenomenon. Summoned at the behest of a dark cabalist or as a curse at the end of a twisted sacrificial rite, the Rain immediately sweeps into an area, destroying crops and spreading negative energy as it goes. The black and purple clouds that carry the rain quickly sweep in from over the horizon to cover the entire sky, casting everything in a deep twilight. Then, amid flashes of lightning, large drops of steaming and hissing green rain drops fall, burning organic matter everywhere they land. The rain often covers a fairly large area, and can last from just a few minutes to a matter of days. Worst of all, victims of the rain (whether humanoids, monsters, or animals) quickly raise as skeletons as the remains of their burnt flesh fall from their acid-etched bones.
 Game Rules
Area: 10d10 square miles (smaller, more contained areas are possible, but rare).
Crushing Darkness: The dark clouds shroud the area in a deep, evil twilight that reduces all visibility by 1/2 normal and causes all organic creatures in the area to suffer a -1 morale check to all attack rolls, ability checks, skill checks, and saves while within the area of effect, as negative energy itself saps their motivation and strength of will. Paladins and clerics of evil deities are immune to the morale penalty effect.
Acid Rain: The rain deals one point of acid damage each round to every organic being caught in the open. Structures, constructs, and undead are not affected.
Vile Reanimation: All creatures that die as a result of the rain rise 1d4+1 rounds later as a skeleton of the appropriate size. Creatures lacking skeletal systems (including oozes, some vermin, some animals, and other creatures, as evaluated by the DM as the situation warrants) are not reanimated.