Wights are shadow skeletons that are faster and stronger than regular Skeletons. They are the bony remains of fallen warriors brought to life and as such they fight more aggressively. They start appearing on Hard difficulty. They are elite enemies that cannot be grabbed by a Dwarf under normal conditions.
They have notably higher health then regular skeletons, have a chance to freeze with their attacks, and can wield cross bows.
One quest, Best of the Dead, requires you to find and kill five of these shadow skeletons.