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A grue is a fictional predator that dwells in the dark. The word was first used in modern times as a fictional predator in Jack Vance's Dying Earth universe (described as being part "ocular bat", part "unusual hoon" and part man).
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 devour adventurers, making it impossible to explore the game's dark areas without a light source. The grue subsequently appeared in other Infocom games.
Zork lore 
The first mention of grues in the Zork games is the line
- It is pitch black. You are likely to be eaten by a grue.
Further investigation reveals more about them:
- > 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.
Grues were invented to force the player to solve light-related puzzles. 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 they will be caught and eaten by a grue. In later games, in certain situations, it is also possible to be eaten by a grue as a consequence of only standing in the dark.
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 this kind of game over was inappropriate for some places unlikely to have pits, such as the attic of a house, and with no corroborating evidence such as holes in the room directly below.
Grues have been featured in each of the Zork games (with the possible exception of Enchanter) and many other of Infocom's games, becoming a company trademark or in-joke, often referred to with the stock phrases of "slavering fangs", "razor-sharp claws" and "horrible gurgling noises". The science fiction title Starcross reuses both the "You are likely to be eaten by a grue" line and the grue's description. Additionally, Planetfall makes reference to grues having been unwittingly taken from their home planet (which is implied to be the world on which Zork takes place) and introduced to Earth by the alien ship in Starcross, then spread around the galaxy alongside man and become a universal pest for human civilizations. The term is also mentioned in other contexts, such as a racehorse named "Lurking Grue" in the murder mystery game Suspect.
As time went on, the games allowed closer views of grues — Wishbringer allows the player to stumble upon a baby grue and look at it before its parents return. Zork: The Undiscovered Underground, released 12 years later, lets the player character see a grue and states that he/she is the first to do so. Spellbreaker had the magical plane of darkness almost entirely populated by grues and forced the player to survive by using magic to take the form of one of the beasts.
One of the repeated references in Zork's backstory was to an ancient king who directly fought grues in combat; this feat would not be repeated until the interactive fiction/RPG hybrid Beyond Zork, which allows a player of a sufficiently level and with certain items to kill attacking grues. Zork: The Undiscovered Underground featured an extended reference to a line in Zork III about "a whole convention of grues" by having the player infiltrate a literal grue convention, complete with lectures, entertainment and souvenirs.
That game was the first to give a detailed description of how grues looked, having the player disguise themselves as a grue after seeing one and noting that it had a "fish-mouthed head, razor-sharp claws and glowing fur all over". (The reference to "glowing fur" appears to be a mistaken interpretation of Sorcerer describing a grue glowing after a light spell has been cast on it — although Spellbreaker does mention that grues' eyes give off a very small amount of light that lets them navigate in darkness.) However, an actual illustration of a grue had been seen previously, in one of Steve Meretzky's Zork gamebooks, which included a section where the protagonists see a grue face-to-face before being eaten by it, presumably as a way to make the book attractive to Zork fans. Presumably these are not the only instances in the Zork games when grues have been seen — one event in Sorcerer has the player finding a Frobozz Magic Company secret prototype "anti-grue kit" containing a grue costume which the player can use to remain among grues unharmed. (The player in Zork: The Undiscovered Underground replicates this feat, albeit imperfectly.)
There is a running gag of mostly failed attempts to find other means of protection than light against grues, most famously in Zork II where a can of Frobozz Magic Grue Repellent was included as a nearly useless red herring. In Zork III, the Magic Grue Repellent functions more like the player might expect, and lasts for several turns.
The power of light as an Achilles' heel for grues is inconsistently given — some games imply that grues find ordinary levels of light painful but can nonetheless survive them; Zork: The Undiscovered Underground has a grue caught in the light spontaneously combust; and in the Zork Trilogy, the player carries around a sword that glows blue when near danger, but grues can kill a player even when it glows brightly.
The modern graphical adventure games in the Zork series continue references to grues, such as gurgling and growling sound effects. A possible parody of the concept appeared in one puzzle in Return To Zork, in which turning the light off in a hotel bedroom creates the danger of a grue attack; the only solution is to place a piece of lightly glowing rock on the nightstand, providing only so much light that it is still possible to sleep. Zork Nemesis continues a running gag about failed attempts to capture or domesticate grues by including a book with an illustration captioned "The Grue In Its Natural Habitat" (a blank black square). Zork Grand Inquisitor added "Grue, Fire, Water", a variant of Rock, Paper, Scissors wherein "Grue drinks water, water douses fire, and fire scares grue."
In the fourth Zork game, Beyond Zork, an evil being called an "Ur-grue" is introduced as the primary villain. Though similar in name, the Ur-grue is significantly different from the classic grue, being more akin to an evil god of darkness than a simple predatory monster.
Again, Scandinavian and German language may account for this etymology, where the Ur- prefix is used to signify origins. An Ur-grue would be the mother/father of all grues, or possibly the first grue.
In popular culture 
Grues are a common reference in hacker culture or among computer-savvy people. They have cropped up in other fantasy realms, though rarely, as they are seen as being strongly attached to the Zork universe, Infocom and the medium of interactive fiction in general. For this reason many modern interactive fiction works make extensive in-jokes referencing grues; one of the more extensive parodies is a work called Enlightenment, which takes place in a Zork-like universe where the protagonist has overloaded themselves with an abundance of light sources—suddenly finding themselves in need of help from grues to defeat a troll, they are forced to find a way to extinguish them all.
Grues make an appearance in Dungeons & Dragons. Aside from a reference to their being "born in places of darkness" on the Inner Planes and a general sense of shapeless menace, they have very little in common with their Infocom namesakes, despite having been introduced soon after the first Zork games and presumably having been inspired by them. The same creature also existed in the Gateway Bestiary, written for the second edition of the game, but was not at that time given the name "grue".
Nerdcore rapper MC Frontalot's song "It Is Pitch Dark" refers to various elements of classic text-based adventure games, including Zork. Its refrain is "You are likely to be eaten by a grue. If this predicament seems particularly cruel, consider whose fault it could be: not a torch or a match in your inventory."
They have been often mentioned in the comic strip User Friendly with the character A.J. Garret having a phobia of them and doing everything he can to avoid them and the dark places they inhabit. He refuses to listen to the comments of his coworkers stating that grues do not exist. This led to the title of the fourth compilation book, Even Grues Get Full, from a story arc involving him getting scared of grues while trying to move into a dark abandoned missile silo.
On IGN's list of the Top 100 Video Game Villains of All Time, the grue was listed as number 46. When summing up the creature and the development behind it IGN wrote, "The grue's presence may have been a handy solution to a very particular problem in the game design, but it has grown far beyond being a mere gameplay convenience to become one of the chief boogiemen in the early history of video games."
An unseen monster known as the grue appears in the survival simulation game Don't Starve. Players who stand in complete darkness first hear growling sounds, then the player character is attacked repeatedly so long as they remain in the darkness.
- Vance, Jack. The Eyes of the Overworld. Ace. p. 37. "This is the skull-stone of a grue, and at this moment trembles with force." Probable first occurrence in print of the term grue denoting a living creature would have been this quote in this initial serialization of this story/chapter, "Cil", in The Magazine of Fantasy and Science Fiction in 1965, although the phrase "It gave me the grues" (i.e., a feeling of fearful revulsion, short for "gruesome") occurs in the 1934 murder mystery The Nine Tailors by Dorothy L. Sayers.
- Vance, Jack. The Eyes of the Overworld. Ace. p. 143. "Grue: man, ocular bat, the unusual hoon."
- The Best Monsters in Gaming: Grue Gamespot, archived October 28, 2007 from the original
- Top 100 Videogame Villains: Grue is number 46 IGN