I had once created this enemy for an Earth-bound setting involving the old White Wolf Street Fighter setting, but with modifications and additions it becomes excellent to use with the 40k setting. I have a number of backgrounds for native defenders and their powers and such, but that's all on my blackberry. Once I figure out how to sync my phone to my computer I can add them (once I synched to Yahoo, but that's no longer working).
I have few hard and fast rules right now. More fluff than anything. A good scale would be Skalar and his Hands are about a Primarch strength. Overlords would be somewhat stronger than a Chapter Master, and then everything inbetween for their forces.