Hmm.. scary is hard to pull off on models. I think there are some ghost like necrons here on dakka painting in only black and
OSL green.
The egyptian look is popular otherwise but not so very scary.
Personally I always loved these
http://www.hivefleetmoloch.com/ if you go to the gallery and check prey species.
Well maybe not that many ideas, but at least something to get the ball rolling