Can be found anywhere outside of Protection Zones, but more common in swampy areas.
By stepping on one of these you will get the Poisoned status and take 5 points of damage on contact. Then you will lose damage over time, starting at 5 damage per turn for a total of 4x5+5x4+7x3+10x2+19x1=100 total damage. Its effects can be cured by the Antidote spell or Antidote Runes and the gas can be removed by using a Destroy Field Rune on it.
Looks the same as Plant Poison.
See also: Slime (Liquid), Earth Damage, and Poisoned