Deadly Lichen is a bright red color, and is only created from a potion of creeping death or upon death of enemies with the infested mutation. It will poison any non-inanimate creature that stands on it. Levitating or flying creatures are not affected. The poison lasts for a maximum of 5 turns, so in general crossing a patch x squares wide will do x+5 points of damage with no regeneration possible during the traversal. Monsters (enemy or allied) will not cross deadly lichen unless they become completely boxed in, at which point they will take the shortest path to a safe square.
It grows to adjacent squares at a rate approximately one square/10 turns, potentially spreading across a complete dungeon floor. Certain terrain features block the spread, including doors and water. It is very flammable, so setting a patch on fire will generally spread throughout an entire connected mass to destroy it.