Deadly Hazard (Levels 5–10) or Nuisance Hazard (Levels 11–16)
Brown mold resembles a furry, light-brown carpet. This fungus feeds on warmth, drawing heat from anything around itself. One patch of brown mold covers a 10-foot square, and the temperature within 30 feet of it is always frigid.
When a creature enters a space within 5 feet of the mold for the first time on a turn or starts its turn there, the creature makes a DC 12 Constitution saving throw, taking 22 (4d10) Cold damage on a failed save or half as much damage on a successful one.
Brown mold has Immunity to Fire damage, and any source of fire brought within 5 feet of it causes the mold to instantly expand across a surface and toward the fire, creating a new patch that covers a 10-foot square. A patch of brown mold exposed to an effect that deals any amount of Cold damage is destroyed instantly.