Page "Grue (monster)" Paragraph 17

The actual reason light acts as such a potent Achilles ' heel for grues is inconsistently given — some games imply that grues find levels of light ordinary for humans to be intolerably, blindingly painful but can nonetheless survive it ( such as in Planetfall, where an obviously grue-like creature exists in a lit laboratory, " squinting and cursing at the light " — although the laboratory contains other creatures that are clearly mutants, so it is possible that this grue is simply a mutant that is able to survive in a lighted area.

) Zork: The Undiscovered Underground goes to the other extreme, having a grue caught in the light spontaneously combust on the spot.

This latter explanation seems closer to the canon established by the main Infocom game series, since in Spellbreaker, if the player is shapeshifted into a grue and remains in a lit area for too long the light eventually kills him / her ( and it is implied that the amount of light to which he or she is exposed is so faint as to be invisible to human eyes ).

However, in the Zork Trilogy, the player carries around an Elven sword that glows whenever he or she is near danger, the glow being described as a faint blue glow ( when one room away from a dangerous creature ) or a bright blue glow ( when in the same room ).

The sword glows in response to the proximity of any dangerous creature, whether a grue or of some other type, and grues can still kill a player when his sword is glowing, even very brightly ; therefore, either a ) there is variation between games as to how much light a grue can safely encounter or b ) only light having one or more properties that the sword's light does not exhibit ( e. g., a wavelength / frequency / color within a specific range not including that of the sword's light ) is harmful to grues.