Yep. A genestealer cult.
My friends and I started a HoR kill team campaign (their ruleset is amazing, you should definitely check it) and they have a codex for genestealer cults! I decided to start one up and go crazy on the fluff, claiming the stealers in question are of the Ymgarl variety and their targets had been Exodite eldar clinging on to a human settled world.
Then the tyranids arrived, inevitably, and we survived to scavenge a bunch of different equipment. We're starving though, and that's where the campaign begins. Started my test model today and nearly got it finished. More work needed on the blue cloth and tentacles.
Dark Angels. The Rock. Lion El "Dwayne" Jonson. I can't be the first person to have thought of this.