Moon

The moon is that big round thing in the sky that only comes out a night. Technically, it doesn't "come out" anywhere... it's pretty much always there, but it's visibility from Earth is dependent upon which phase the moon is in, which is referred to as it's lunar cycle. In horror fiction, the moon really only serves one purpose. It is a prominent plot device in stories involving werewolves. Classically, whenever the moon is in full visibility (a full moon), a person who is cursed with lycanthropy will transform into a half-human/half-wolf creature.