A lichen is a fungus-based monster that appears in Nethack, and is one of the easiest in the game. They are extremely slow and incapable of dealing any actual damage; their main attack is a sticky attack which holds you in one place, but this is trivial considering they are easy to beat. However, the sticky attack does prevent you from attacking any other target except the lichen, which wastes that turn ("You cannot escape from the lichen!") and forces you to focus on killing it first.
Lichen corpses, unlike most corpses, never rot - but they do become old for the purposes of sacrificing. They can be carried indefinitely as normal comestibles which are always safe to eat; they are also valid non-meat food choices for vegans and vegetarians, and can be used to tame horses. Dipping a lichen corpse into a potion of acid causes it to 'turn red around the edges', or orange if the potion is somehow diluted. This does not consume the potion.
Lichen corpses have the same weight as a food ration for a quarter of the nutrition, and should ideally be eaten first to free up carrying capacity.
Lichen were added in NetHack 3.3.0.
NetHack 3.6.0 added messages when a lichen corpse is dipped in acid.
While lichen do not have variants in vanilla Nethack, the arch-lichen patch by Nodey for NetHack 3.4.3 allows the variant known as the arch-lichen (based off the arch-lich) to be patched into a game. Slash'EM Extended has this patch implemented by default with some changes, and also introduces many stronger variants of the lichen, including an actual master lichen.
In increasing order of strength, the variants encountered are: aggressive lichen, water lichen, poison lichen, venom lichen, ore lichen, patched lichen, master lichen, grandmaster lichen, sensei lichen and arch-lichen.
Lichens are organisms that originate from the filaments of multiple species of fungi, and are known for being among the oldest living organisms. They can occur in many environmental conditions, including several extreme ones, and can grow on almost any surface. Real-life litmus tests use dyes extracted from lichens as a pH indicator.
The chamber was of unhewn rock, round, as near as might
be, eighteen or twenty feet across, and gay with rich
variety of fern and moss and lichen. The fern was in
its winter still, or coiling for the spring-tide; but
moss was in abundant life, some feathering, and some
gobleted, and some with fringe of red to it.