Comes from the Old English verb for "shudder" (presumably the origin of the modern word "gruesome" as well; literally "shudder-some"). Grues are creepy, sinister creatures from the Zork series, the oldest and most influential of the old text-based adventure games. They're hardly ever seen because they despise light and only emerge in areas of complete darkness, where they prey upon adventurers and enchanters. Easily pissed off and vicious, they're repelled by even the faintest glow of light; meaning that a simple torch can be enough to keep the grues at bay. They're mysterious; short of their glaring eyes and sharp fangs-and-teeth, nobody knows what grues look like, exactly. They're known for their constant slaving and gurgling, and especially their nasty tempers.

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;...

A fearsome creature that dwells in the darkest corners of the great underground empire. It has a taste for adventurers. It's biggest fear is light- the feeblest of torches can keep a grue at bay

The combination of great and true.