Russet Mold
Generic Hazard
Few know for sure where russet mold came from. One historical account tells of adventurers in a mountain range discovering russet mold and vegepygmies in a peculiar metal dungeon full of strange life. Another story says that explorers found russet mold in a crater left by a falling star, with vegepygmies infesting the forest nearby.
The mold is found only in places that are dark, warm, and wet. Russet mold that spreads out across a metal object can be mistaken for rust, and a successful DC 15 Intelligence (Nature) or Wisdom (Survival) check is required to identify it.
Effects of the Mold
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 is poisoned. While poisoned in this way, the creature takes 7 (2d6) poison damage at the start of each of its turns. 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 dies. If the dead creature is a Beast, a Giant, or a Humanoid, one or more vegepygmies emerge from its body 24 hours later: one 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.
Destroying the Mold
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 magic 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.
Source: Volo’s Guide to Monsters p. 196, Mordenkainen Presents: Monsters of the Multiverse p. 252