These fumes rise from volcanic vents, venomous incense—or both, in the case of rare sacrifices involving yuan-ti priests that expect to rise again after dying within the vent where the sacrifice is conducted.
Effect: These squares are lightly obscured, and creatures in them must hold their breath or suffocate. (See “Suffocation".) Creatures that have immunity or resistance to poison are immune to this effect.
Usage: Add complexity to an encounter by introducing the vapors after a fight starts. Or have a creature that has poison resistance use the vapors to hide in while it attacks from concealment.
Published in The Plane Above, page(s) 86.