A simple idea for a darkness inspiratnal. Acting much like a tumble weed, only faster, the will-o'-wisp would be invisable unless in darkness as well as on the map (rendered as a small but intence light sorce). Due to needing the dark to see it it could intice players to remove their torches (or client side brightness) in order to chase them, and while not dangerus themselves they might end up leading players into other mosters while they are distracted with the chase.
The Will-o'-wisp might also be useful to craft ever burning lights for mines or around darkness built towns/bases - thus creating a need for such a tricky little light.