Then it becomes a matter of aesthetics as far as characters go. I've always found the Covenant to be plausible and aesthetically pleasing as Cylon Centurion or Klingon type characters ie. the race that humanity meets during the outward expansion into the galaxy while they are leaving this world behind and that come into an unexpected war against. However, I feel that the Helghast are far less believable and much less iconic than the creatures that make up The Covenant as think that actual soliders from Hell would tend to be more like Clive Barker's vision of cenobites instead of the painting of Darth Vader that is in the Denver International Airport.
I've heard that the creatures that make up the enemy in The Conduit fall into the alien conspiracy type opponents that might be something like the nemesis in the Area - 51 games, so I think they will probably end up being more believable than the Helghast but not as believable as The Covenant.
Heavens to Murgatoids.
