This race was technically Human once before. Leaving to try colonizing efforts on the planet of Helghan, the humans found living on the world uninhabitable. Lightning storms and harsh atmospheric conditions made it impossible to live on, but after they felt they were betrayed by their former Earth comrades, they revolted and started their own nation.
They chose to go back to this planet to live. Many of the first generation died off but some survived, while their future generations evolved biologically to survive the harsh planet.
Now that is a quick summary, but as Killzone taught us they are not to be trifled with. They are formidable opponents who have mindset that leads them to believe they are superior. They invade and attack other colonies just to assert their dominance.
This race willingly inhabited a harsh planet that is constantly hit by lightning storms, put themselves into a dictatorship, and adapted to everything that was thrown at them. Include the fact they invade other planets, I can see them being a hard race to handle.