Brown mold is a fungus found in underground areas. A patch is typically about 5 feet in diameter, and could be any color from dark brown to pale tan. Brown mold feeds on light and warmth, save only for ultraviolet light (which is why it is found underground, as direct sunlight or other sources of ultraviolet light will cause it 1d4 points of damage per turn).

The area surrounding a patch of brown mold will be noticeably colder, and any living creature standing within 5 feet of a brown mold will suffer 1d8 points of cold damage each round. Exposure to heat (such as a torch, fireball spell, etc.) causes the mold to grow almost instantly. Fires that would otherwise do up to 10 points of damage will cause the patch to add an additional 20 square feet or so in size (approximately doubling its initial size) with each additional 10 points or so adding a similar area. This effectively increases the area of its cold damage. Any type of fire or cold attack will not harm this mold, but attacks with weapons or lightning can damage it (though weapon attacks may also damage the weapon, if the relatively thin layer of mold lies over metal or stone). Exposure to ultraviolet light (including sunlight) renders a brown mold dormant, negating its cold attack completely.