You are likely to be eaten by a grue

Grues in popular fiction

A grue is a fictional predator that dwells in the dark. The word was first used in modern times as a fictional predator from Jack Vance's Dying Earth universe (described as being part "ocular bat", part "unusual hoon" and part man).
Vance probably took the name from an archaic/dialectal English verb derived from a Scandinavian word meaning to feel horror, shudder (OED), now most commonly encountered in the word "gruesome" (Danish: "grufuld", Swedish: "gruvlig", Norwegian: "grufull").

Robert Louis Stevenson mentions the grue in the short story "The Waif Woman" (1916) which takes place in Iceland. As a greedy woman lies in bed gloating over the fine clothes she has wrongfully acquired from a recently deceased woman, "a grue took hold upon her flesh… and the terror of death upon her soul".

Grues in Zork

Dave Lebling introduced a similar monster, whose name was borrowed from Vance's grues, into the interactive fiction computer game Zork, published by Infocom. Zork's grues fear light and are ravenous devourers of adventurers, making it impossible to explore the game's dark areas without a light source. The grue subsequently appeared in other Infocom games.

Due to Zork's prominent position in hacker history and lore, its grues have served as models for monsters in many subsequent games, such as roguelike games and MUDs.

Please provide examples to illustrate the grue's prominent position in hacker history and lore

The first mention of grues in the Zork games is the following ominous line:

It is pitch black. You are likely to be eaten by a grue.
Further investigation will reveal more about their nature:
> what is a grue?
The grue is a sinister, lurking presence in the dark places of the earth. Its favorite diet is adventurers, but its insatiable appetite is tempered by its fear of light. No grue has ever been seen by the light of day, and few have survived its fearsome jaws to tell the tale.

This warning is not to be taken lightly. If the player attempts to continue moving through a dark place rather than returning to a lit area or activating a light source, there is a high probability he will be caught and eaten by a grue. Originally, grues were not a threat as long as one remained still and didn't leave one's location, but in later games it has been possible, in certain situations, to be eaten by a grue simply by waiting around in the dark.

Grues were invented to limit players' options when faced with unlit areas. If a player should attempt to blunder about in the darkness in hopes of achieving whatever goal first brought them there, the presence of grues ensures that they will fail, forcing the player to solve any light-related puzzles first.

Zork's predecessor, Colossal Cave Adventure, used bottomless pits to achieve the same result, but when early versions of Zork adopted this practice, it was realized that pits were appearing in unlikely places, such as the attic of a house, and with no corroborating evidence elsewhere (such as holes in the ceiling--or floor!--in the room directly below). Thus, Dave Lebling envisioned a wandering, light-fearing monster that could do the job of the bottomless pits, and, taking the name from Vance's work as having the right connotations, introduced grues in the next version of Zork.

The version update document made a humorous reference to the "dungeon maintainers" painstakingly filling up the bottomless pits and restocking the dungeon with grues. Years later, the Zork prequel game, Zork Zero, would feature the protagonist doing exactly that: forced to use a magic device to seal up the realm's bottomless pits that are blocking his path, he unwittingly forces out the myriad colonies of grues that have been nesting there, leaving them to wander the underground caverns searching for food.