The setting is much different from the basic idea that comes with fantasy based games for the simple fact that everything that happens in this game is taken place in "our" very own universe. Approximately 5 billion year from now, our sun will go through its final stages of existence in our solar system where it will expand outward to engulf a majority of our solar system.
At this point humanity was at the pinnacle of science and technology to the point that space travel was as difficult as changing a tire on a car. With that being the case, it still didn't make things easier to access for everyone. So when the danger arose from the death of their star, most of the planet left to live among the stars to find new homes in the cosmos while 20% of the populous was left with fortified bunkers made to withstand the onslaught of the sun for 1000's of generations to pass, to live in their domes like slaves till the end of time.
Though, the expansion of the sun wasn't the end of the planet as they had first believed. An unknown force that emanated from earth itself shielded the blue globe from the harmful effects of the star as it ravaged the rest of the solar system. Though during this time, the earth itself went through drastic changes. The wild life altered, changed, became sentient, developed cultures, and thrived. The 5% of the population unable to find shelter in the bunkers in time mutated. Those that hid among the caves grew larger and fouler. There structures hardened and growing fierce beast like jawlines, as the humans that remained in the shielded cities where given abnormally long life and slender forms plus keen minds. All the while the humans in the bunkers remained oblivious to the changes the world took outside of their walls.
Everything from that point on changed to make the world what it is known to be as for them. This period of change was known as the Cleansing