The year is 2120, a century ago, the Chaos God of Destruction, Sire unleashed most of his chaos energy to rip apart the space-time barrier on Earth. With the space-times barrier gone, The Chaos God sent his endless hordes of monsters and generals to attack humankind before falling into a deep sleep. Seven massive rifts tore the air open in heavily populated areas around the world. Hordes of Ratmen, Orcs,and Skeleton soldiers and other monsters from what thought to be fiction rushed out of the gates brutally killing everyone and destroying everything in sight. Enormous amounts of energy leaked out of the gates terraforming the air so that it is suitable for monsters. This energy would later be known to the world as mana. This horrific event is stamped in human history as The Invasion of Chaos.
The countries and militaries of the old world did what they could, with every country spending all their resources in researching weapons to defeat the horde. With options and time running short, the superpower countries tried dropping nuclear bombs on the horde. However they just kept coming. Bigger, Faster, Stronger monsters that resisted the weapons of the 21st century passed through the rift. The Invasion of Chaos lasted six months with nearly 90% of the population wiped out worldwide. Countries toppled, cities destroyed, billions dead. It was nearly the end of the human race as we know it.
However in times of desperation, it seemed like humankind's prayers had been answered. Humans capable of awakening their potential to harness mana started popping up in the last bastions cities of Earth. These hunters banded together and fought back against the monsters of Chaos injuring/killing several generals to established borders from the seven mega rifts. Civilization can start rebuilding however life could never be peaceful like before. Although the many waves of monster had stopped attacking, the Invasion didn't.
A hundred years later, the world is still dealing with these monsters and eventually the Chaos God will wake up and all hell will break loose again.