Russet Mold

From Volo's Guide to Monsters, page 196.

Hazard

The fungus known as russet mold is reddish-brown in color and found only in places that are dark, warm, and wet. Russet mold that spreads out across a metal object can be mistaken for natural rust, and a successful DC 15 Intelligence (Nature) or Wisdom (Survival) check is required to identify it accurately by sight in such a case.

Any creature that comes within 5 feet of russet mold must make a DC 13 Constitution saving throw as the mold emits a puff of spores. On a failed save, the creature becomes poisoned. While poisoned in this way, the creature takes 7 (2d6) poison damage at the start of each of its turns, sprouting mold as it takes damage. The creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success. Any magic that neutralizes poison or cures disease kills the infestation. A creature reduced to 0 hit points by the mold's poison damage dies. If the dead creature is a beast, a giant, or a humanoid, one or more newborn vegepygmies emerge from its body 24 hours later: one newborn from a Small corpse, two from a Medium corpse, four from a Large corpse, eight from a Huge corpse, or sixteen from a Gargantuan corpse.

Russet mold can be hard to kill, since weapons and most types of damage do it no harm. Effects that deal acid, necrotic, or radiant damage kill 1 square foot of russet mold per 1 damage dealt. A pound of salt, a gallon of alcohol, or a magical effect that cures disease kills russet mold in a square area that is 10 feet on a side. Sunlight kills any russet mold in the light's area.