Traveller

What is the narrative purpose of the Black Ships in the Traveller mythology? When were they first encountered? Are they supposed to represent an ancient threat, like B5's Shadows? Are they technologically superior to the Third Imperium and thus an invasion risk, like Star Trek's Borg? Are they something you throw at your players when they're "doing it wrong" to discourage a given course of action? Help me understand where they fit into the setting.