Zork:Grue/summary

From Zork

Grues are dangerous creatures that have never been seen by the light of day. In fact, 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.

To defeat a grue, you can simply give it light. As no grues can be seen by the light of day, they simply disappear in a puff of logic, so you are free to move.