Blood Midge Cloud
A swarm of biting insects fills the area.
Effect: The squares occupied by the cloud are lightly obscured, and the cloud is difficult terrain. If no creature is in the cloud when a creature starts its turn within 3 squares of a blood midge cloud, the cloud moves 5 squares and attempts to cover as many living creatures as possible. If an area or close attack includes squares occupied by the cloud, the cloud is destroyed in those squares. If a cloud is broken into smaller pieces by an attack, each piece acts as a separate terrain feature.
Usage: A blood midge cloud is interesting because it is mobile obscured terrain that can move to engulf the greatest number of creatures. The concealment granted by the cloud helps anyone in the cloud, and it’s particularly beneficial to ranged attackers, who can fire out of it easily.
Published in Dungeon Master's Guide 2, page(s) 58.