A lichen is a fungus-based monster that appears in Nethack, and is one of the easiest monsters to kill in the game. Their only attack is a sticky attack that holds the player in place briefly, but otherwise does no damage.
Lichen are extremely slow and easily beaten even by beginning characters; however, their 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.
Unlike most corpses, lichen corpses never rot (though they do become old for the purposes of sacrifice), and can be carried indefinitely as normal comestibles which are always safe to eat. They provide 200 nutrition, but weigh the same as a food ration, so they should ideally be eaten first to free up carrying capacity. Lichen corpses are also valid food choices for vegans and vegetarians, and can be used to tame horses.
Lichen were added in NetHack 3.3.0.
NetHack 3.6.0 added messages when a lichen corpse is dipped in acid.
Slash'EM Extended has the arch-lichen 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.
Lichen are composite organisms that arise from algae or cyanobacteria living among filaments of multiple fungi species. They can occur in many environmental conditions, including several extreme ones, and can grow on almost any surface. They also have a history of being used in real life as food, hence their potential use as sources of long-term food in NetHack. Real-life litmus tests use dyes extracted from lichens as a pH indicator, which is why lichen corpses in NetHack turn red when dipped into potions of acid.
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.