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.
welp time to bring my fairy bell and orb of light
