STAR TREK NG: S2 Ep2, "
Where Silence Has Lease"
The Enterprise is sucked into a dimensionless void controlled by a powerful and sociopathic entity, NAGILUM, that performs a deadly rats-in-a-maze kind of experiment on it. At the end of this episode Picard vows that when they meet again, it'll be among the stars (and not in the void), suggesting a future Nagilum appearance. But Nagilum only appears in this episode and, as far as I know, doesn't appear in any other ST spin off. One wonders whether other omnipotent or seemingly omnipotent entities like the Q or Organians ever had dealings with Nagilum.
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