Low ghosts are so old they've almost faded away. Or, some say they're just the spirits of those most powerless and forgotten. Either way they seem incapable of harming someone, only possessing them. A low ghost will follow a person, moaning in the shadows, until they take possession of them (can attempt each round). Once they possess a person they wander off and lose themselves in the darkness. Even their ability to possess is weak, though; and the more souls nearby the less likely it becomes (every person touching a target gives it bonus to save).
More a distraction than a direct threat, but I had an image come to mind while falling asleep one night of a party all holding hands with these things all around them and then coming to a ladder or rope down . . .