Carpet of Serpents
These patches of ground are covered with swarms of serpents (or other venomous creatures) that dart in and out of various hidden dens.
Effect: A carpet of serpents is difficult terrain. When a creature starts its turn in a carpet of serpents area, it takes 5 poison damage per tier, or 10 poison damage per tier if it is prone.
Usage: Combine a carpet of serpents with creatures that have poison resistance or immunity, or as part of a trap that drops the characters into a chamber that has this feature.
Published in The Plane Above, page(s) 86.