In “Where Silence Has Lease,” Nagilum traps the Enterprise with plans to experiment on at least 1/3 of the crew, so that it can study death. Does this imply that Nagilum is immortal (incapable of dying from “old age” or natural causes, but not accident or murder)? Or perhaps its species is simply extremely long-lived, and so none of them had yet died at the point when Nagilum met Picard and company.
Could Nagilum be eternal, existing before the ”Beginning?” This seems unlikely, as only the Q in Star Trek have claimed such a thing about themselves. Though Nagilum very well could be a “brute fact,” without any explanation whatever.