Lichen

Lichen comes from a potion of creeping death or can be spawned by enemies with the infested mutation upon death. It will poison any non-inanimate creature that stands on it. Levitating or flying creature are not affected. The poison lasts for a maximum of 5 turns. It also slowly spreads across the dungeon floor, but certain other terrain features stop it, including doors and water. Setting it on fire will destroy it.