In the game's feedback display itself, the grues were often a warning that a user should start looking for a light source... badly. "You have entered a dark area. You will likely be eaten by a grue" was a common description in the Zork adventure games; adventurers going underground would be wise to bring a good supply of oil for a lantern. Once the lights go out, the grues swarm all over you (ripping you apart before the game gives you a death message, like "Grues eat well tonight"). Infocom's official description of the "grue" (notice the lack of plural; it was unclear in the earlier Zork instalments whether there was only *one* grue, lurking as a sinister presence in the underground's darkness... or more then one) is:
"The grue, according to scholars of the Great Underground Empire, is a sinister, lurking presence in the dark places of the earth. Its favourite diet is either adventurers or enchanters, but its insatiable appetite is tempered by its extreme fear of light. No grues have ever been seen by the light of day, and only a few have been observed in their underground lairs. Of those who have seen grues, few have survived their fearsome jaws to tell the tale. Grues have sharp claws and fangs, and an uncontrollable tendency to slaver and gurgle. They are certainly the most evil-tempered of all creatures; to say they are touchy is a dangerous understatement. "Sour as a grue" is a common expression, even among themselves."
User #1 (types in): "head downwards"
Screen: "You walk into the darkness. You will likely surely be eaten by a grue if you don't find a light source."
User #2 (talking to first user): "Dude, you're gonna get eaten by grues..."
User #1: "Don't worry, I brought a lantern." *types in* "take out lantern*
Screen: "You left the crypt door open. A draft blows through, putting out your lantern."
User #1: "Oh shits, I knew I should've brought a hooded lantern."
User #2: "Quick, light it again!"
User #1: "Can't, man. No oil or matches."
Screen: "Another wind picks up and closes the crypt's door, blocking out the moonlight."
User #1: "Oh man, no matches or anything...? We are *so* fucked..."
Screen: "It's nice and dark in here. Just the way the grues like it..." *screen goes black, followed by nasty growling sounds*
User #1: "Son-of-a-bitch... remind me to stick a rock in the door to keep it from blowing shut next time."