Grue, Vashta Nerada, Devourer in The Dark.
Many children fear night-time and the dark spaces of the world, but most learn to dismiss this fear as they grow to adulthood. This is a potentially deadly error. When entering a place occupied by a grue, a lack of illumination spells near-certain death. All that is necessary to ensure safety, however, is to bring forth a source of visible light. It is of special note that beings with some ability to see in utter darkness are not thereby made safe. The grue will remain quite unviewable to persons of this proclivity, yet its deadly efficacy will be unabated.
None know what manner of physical form this creature possesses, for upon exposure to visible light it is instantly dismissed. Sages ponder the nature of this banishment, but most agree it has to do with the inherent shadows present within the folds of reality. Some have heard sounds of slavering and the gnashing of a great maw just before the spark of a flint lit their lantern, yet there are others who report only the clattering of an object or two before their newly lighted torch revealed a bit of gear which belonged to a now missing compatriot.
Statistics - a decidedly deadly beast to encounter, the grue has more in common with a devilish trap than a typical creature. If in an area occupied or frequented by grues, and unfortunate enough to be without a source of visible light, a character in utter darkness for more than one round must save versus paralysis or be utterly eliminated. 1d4 items carried on their person, determined randomly, may fall to the ground and remain behind. The grue will only devour one character per round, determined randomly from those present. Those devoured by a grue may be retrieved by a wish, but no other means is sufficient.