A powerful being living in a vast black void who trapped the Enterprise to study death by killing part of the crew.
Nagilum was a being that dwelled within a region of space that looked like an empty hole, devoid of stars. It created illusions to test the crew, including a phantom copy of a Federation starship, and eventually appeared as a face on the viewscreen. It knew nothing of mortality and showed little regard for lesser lifeforms.
In Where Silence Has Lease, Nagilum announced it would kill a third to half of the crew to learn about death. Rather than submit, Picard set the ship to self-destruct. Nagilum released the Enterprise moments before the countdown ended, claiming it had learned enough.