Head of the Prey

I have this idea that the Greys are advanced humans from an alternative timeline of some kind. Let's say that they developed the ability to travel between alternate universes and interact with the inhabitants of the parallel Earths that they visit. Now imagine that one of these Earths is the seat of the Vitara Intragalactic Hegemony; imagine that the Vitara reacted with hostility to intruders from other histories, that they do not approve of life forms screwing around with the developments of more primitive cultures. Also, keep in mind that the Vitara are predators, and something that evolved on Earth would most certainly be compatible with their digestive systems...