Most common in regions of the Nine Hells and certain volcanic regions of the world, infernal fumes are great clouds of gray smoke that glow red with inner heat. Infernal fumes drift as if alive, apparently seeking concentrations of multiple creatures.
Effect: Squares filled with infernal fumes are lightly obscured. The fumes can move up to 4 squares each round. A creature that ends its turn in a square of infernal fumes takes 5 fire and poison damage per tier and is dazed until the end of its next turn.
Usage: Infernal fumes are an effective way to chase player characters around an encounter area. This terrain is similar to a blood midge cloud, but has a significant harmful effect to characters and monsters inside it. Few monsters, even devils, are immune to the effect of infernal fumes, so they challenge both sides of a fight equally. Creatures that remain outside the cloud are hindered in their attempts to attack creatures inside the cloud as a result of the concealment it grants.
Published in Dungeon Master's Guide 2, page(s) 60.