While the Assault Fiend (along with the Succubot) was one of the original figure conceptions, the original intent for the boxed set was a straight up special-forces-storm-the-lair-defended-by-conventional-forces battle with a commando section against 12 Nightmares. Rob Prior, the artist who later did the cover art for the Black Box said "You just GOTTA have THAT guy in the basic game."
When I first came up with the idea, I called it a Balrog. Then we saw Tom Frank's conception art. We left it virtually unchanged and adopted the title he had scribbled in the corner: "Assault Fiend, Mk I".