If I remember correctly, Nightspawn was connected to Rifts. Or am I thinking of something else? I tried to get my friends Scott and Bill to play it in the late 90's, but it never got past the planning stages. I would still like to try it.
Nightbane (or Nightspawn) is only tangentially connected to Rifts. Most of the crossover is one way (Nightbane to Rifts). I tend to ignore the Rifts: Dark Conversions book. The Nightbane conversions there tend to be rather silly.