Madness by Firelight

You're sitting around the campfire; it's dark, the moon is high in the sky, the wind is whistling through the trees, and the sense of isolation is pervasive. You hear a twig snap in the darkness, everyone is tense. What's the story you tell your friends to terrify them? What's in the dark, stalking the camp: an ax murderer, a werewolf, or something so far beyond our mortal understanding that just the sight of it will drive each of us insane?