Brown mold was a pale, fur-like fungus that spread in mats across stone and soil, draining warmth from the world around it. A single patch filled a ten-foot square, chilling the air for thirty feet in every direction and turning even temperate chambers into frigid hazards. Any creature that strayed too close was struck with a sudden and unnatural cold, suffering waves of freezing pain that could sap strength and life within moments. Entire caves, ruins, or dungeons could be rendered uninhabitable once brown mold took root, its hunger for heat warping the environment until only frost and silence remained.
The mold had peculiar properties that made it both dangerous and fragile. Fire could not harm it, and in fact caused it to spread at once, doubling its reach in pursuit of the heat it fed upon. For this reason, torches and flames were perilous to carry near its growths, often turning small patches into sprawling infestations. Cold, however, was its undoing: any strike of frost or spell of winter could destroy it instantly, unraveling its strange balance and leaving behind nothing but brittle remains. These traits made brown mold a deadly trap for the reckless and a careful puzzle for those who sought to clear or control it.