Cloudspores are created by strange mushrooms found in the Underdark. The mushrooms, which are normal terrain, release a thick cloud of spores when creatures move through an area where they grow.
Effect: When a creature enters a square of cloudspore mushrooms, that square becomes lightly obscured (Rules Compendium, page 220) for 5 minutes. Once a square of cloudspores has discharged, it cannot do so again for 24 hours.
Usage: Cloudspores work well with encounters that include lurkers. The lurkers can either remain in the lightly obscured square, or move from one area to another to gain concealment and possible combat advantage. Mixing poisonous spores with cloudspores in later encounters is also a good way to surprise the characters.
Published in Dungeon Master's Guide, page(s) 67.