It’s a reference to an old computer game Zork. Whenever you were in a dark place, they would threaten you with Grues.

The link is kind of busted by the “_”. You can just click grue (monster) at the page it brings you too. The entry is pretty entertaining, actually.

“In their original reference, in Jack Vance’s Dying Earth series, grues are described as being part “ocular bat”, part “uncanny hoon” and part man.”

I’m bored today…