Classic Acid Rain: A half-globe of glass, mounted on the ceiling, is sensitive to smoke and similar particulate matter (such as glitterdust, sand, mist, vapour, and so on). If it detects such a substance, tiny spouts on it open, raining burning acid down upon the room.
Characters caught in the area take 3d4 damage each minute they are exposed, and for another 1d4 minutes AFTER they are exposed unless they wash the acid off immediately.
The rain stops the moment the characters leave the area, and starts again if they walk through with smoke or other such stuff.
At the GM's discretion, the half-globe may be shattered from a distance by rocks, spears, arrows, and so on, dumping the acid all at once, defeating the trap. This trap can otherwise be deactivated through the thief skill of finding and disarming traps, or through certain magics.
It is not triggered by magical fire or light that doesn't render smoke.