They are the Fear of the Unseen, of the Dark and of the Night. They are living shadows, and as such can not act if they are in direct contact with light
Nightlanders have a strange obsession with order. When they are residing in a place, they will move objects around to suit their own incomprehensible version of feng-shui, which can cause great frustration for the current residents.
They are mostly passive, unless someone attempts to undo their work. In that instance, they will become hostile, and possibly rearrange the offender's organs.