Nagilum

Nagilum was a powerful extra-dimensional non-humanoid lifeform who lived in a "hole in space" that was devoid of all matter and energy in the Morgana Quadrant. It encountered the USS Enterprise (NCC-1701-D) in 2365.

In the course of conversations with Nagilum, it called Humans limited, apparently because they were mortal, and it wished to perform experiments on them. It was interested in the concept of "death" and killed Ensign Haskell simply to witness the effect on him. After this it decided to study this aspect of humanity on numerous levels, proposing to kill a large portion, from one third to one half, of the ship's crew.

Captain Jean-Luc Picard refused this and threatened to kill his entire crew via auto-destruct rather than letting them be experimented on, and this made Nagilum let them go. In a final conversation with Picard, Nagilum revealed that it had learned everything that it needed and admitted that while quite different, it agreed with Picard that it shared the trait of curiosity with humanity.