Nagilum was the main antagonist of the Star Trek: The Next Generation episode Where Silence Has Lease.
He voiced by Earl Boen.
History
Nagilum was a formless immortal being with a "vast" intelligence. He was so advanced that even Deanna Troi could not discern his existence.
In 2365 the USS Enterprise encountered him in the Morgana Quadrant and trapped the Enterprise in a region described as a hole in space. There Nagilum subjected the crew of the Enterprise to a number of illusions, including an attacking Romulan warbird and an illusion of the Enterprise's sister ship Yamato.
After Dr. Pulaski correctly surmised that someone was testing them, Nagilum generated a human face to communicate with the Enterprise crew. Realizing that humans had a limited existence, Nagilum decided to study the concept of death, starting by killing the conn officer Ensign Haskell. Nagilum told Picard that he would continue studying the concept, and that it should cost the lives of 1/2 to a 1/3 of the Enterprise crew.
Rather than allow Nagilum to pick off the crew one at a time, Picard and Riker decided to destroy the Enterprise, and activated the self destruct. Watching the crew face death, Nagilum decided he had everything he needed, and released the Enterprise. After being convinced that the Enterprise had truly been set free Picard cancelled the auto destruct sequence.
Nagilum then spoke to Picard, and shared what he had learned - mainly that humans were an aggressive and power hungry race, and that they had nothing in common. Picard noted that they did have one thing in common, a strong sense of curiosity. Nagilum stated that perhaps they would meet again, but Picard told him that it would need to be out amongst the stars. With that Nagilum disappeared, and Riker told Ensign Crusher to steer clear of any other holes in space that might show up.