In the beginning, there existed the Angels, the Demons and the Humans; three races together in one world, all still in their infancy. Each represented a different aspect of growth; Order for the Angels, Chaos for the Demons, with the Humans sitting in the middle with Balance. The Angels existed as a unified race that sought to purge itself of diversity, and thus, of inequality. All grew together, all declined together. None were left behind, but none were allowed to excel. The Demons existed as a wild and unpredictable race, constantly shifting and changing to suit their environment and the energies that flowed around them. They saw everyone as an enemy, even each other, and constantly fought amongst themselves in an attempt to establish dominance. A few exceptional demons reached levels of frightening power, but constantly had to defend themselves against challengers hoping to take the top spot for themselves. The Humans were both and yet neither, always growing but at a slow pace that allowed some to achieve dominance while others faded into obscurity. The Humans, however, had what neither of their physically more powerful cousins possessed; limitless potential. It would prove to be both their greatest asset and their greatest weakness, as it made them an appealing target for the Angels and the Demons.
As the most disciplined of the three races, the Angels' society grew the fastest, and they were the first to become truly aware of the universe around them. Blind to the negative aspects of their culture, the sterility and stasis, they thought themselves superior to all others and set about establishing structure and stability similar to theirs in the world around them. While the Humans were receptive to the teachings of the Angels, even if they didn't entirely understand them, the Demons simply laughed in their faces. Like the Angels, they thought their way of life was the only real option, and delighted in destroying all that the Angels tried to build.
Though unperturbed by the Demons' tauntings, the Angels saw that their violent cousins were interfering with the development of their prodigal Humans, and so sought a peaceful and productive solution to the problem. Finally, the Angels pooled their formidable collective powers and split the world in two, with the Demons in one mirrored dimension and the Humans in the other. With their focus solely on protecting the Humans, however, the Angels neglected to give the Demons their share of the world's dimensional protection; like a house without a roof, the new Demon World was left exposed to the elemental energies of the universe that were so carefully regulated by the insulation protecting the Human World.
The result was an explosive bout of evolution. While the different Demon species had previously only been a few dozen, the sudden influx of cosmic energies combined with their natural ability to adapt to the energies around them splintered them into hundreds, if not thousands, of diverse species. As the lands around them were changed and mutated by the elemental energies, the Demons living in them adapted to those changes, dividing them along lines of not just physical build, but elemental affinity.
While the first weeks were violent and confusing, as brother turned on no-longer-recognizable brother and many lost their sense of self from such drastic changes, the Demons soon took to their new situation with enthusiasm. After all, their race had always thrived on chaos and power, and now they had more of both than they knew what to do with. True, they had experienced a somewhat uncomfortable rebirth, and their new world was harsh and unpredictable, but these trials would allow only the absolute best of their race to rise to the top. And if there was one thing that all the Demons agreed on, it was that only the strongest among them was fit to rule the rest of the pack. What they did not realize, however, was that the best among them would not come from the Demon World itself.
In the Angels' pursuit of eliminating diversity and strife from among them ranks, the Archangel Mundus was placed in the spotlight. Desiring power more than anything else, Mundus shared the Demons' "survival of the fittest" mentality, and when he was unable to convert the Humans to his cause, the Angels expelled him from the Human World and into the Void. Though it was assumed he would perish in the cosmic maelstrom of the Void, Mundus instead did what his demonic counterparts excelled at; he adapted. His body mutating to reflect his corrupt and malevolent soul, he became a Demon in body as well as mind. Though he retained his outward angelic appearance, everything beneath the surface was twisted and perverted, a disgusting amalgam of the cosmic powers he now drew upon. Taking up residence in the Demon World, the chaotic dimension slowly became influenced by his corruptive presence, taking upon an even-more sinister tone than before.
Years passed, and the Demons soon came to realize that they were being left behind by their cousins in the Human World. Under the guidance of the Angels, who had retired to a pocket dimension to observe and protect their subjects from afar, the Humans had made leaps and bounds in development, growing in numbers at an exponential rate. While they did not need to abandon their ideals, the Demons realized that they needed at least some structure and unity to break the cycle of self-destruction that had been stunting their growth.
Their first act was to divide the various Demon species into two categories, based upon power and ability; Lessers and Greaters. The Lessers, while more numerous, were comparatively weaker as the power within their blood was being spread around to a larger population. Alternatively, the Greaters, while fewer in number, were also significantly smarter, stronger and more refined; they had even gone so far as having developed into family groups for mutual support. Thus the Lessers were placed into a peasant class of servitude and their Greater cousins into a dominant class of nobility. And to lead the Greaters would be an Emperor, he who was the most powerful of all Demon kind. It came as little surprise to Mundus that the god-like power he now had at his command allowed him to be relatively unopposed in his rise to such a position of power.
The various Greater families, none wishing to be left out of executive decisions made for their entire race, formed a ruling council that would unite the Demons under Mundus' leadership and put their various talents at his disposal. Together, they began to develop the Demon World around them into a nightmarish reflection of the civilization the Humans in their world had been building for themselves. Though there was still dissent among the ranks, they were united by their desire to prove their superiority to their human and angelic cousins.
As Mundus' throne and the council that worked with him quickly became the focal point of demonic society, a city of sorts sprang up around their base of operations. Dubbed Tartarus by those wishing for convenience, the settlement was considered the capital of the Underworld, housing the homes and headquarters of the more powerful Greater families. As the families and their operations grew, so too did Tartarus, until it became a sprawling metropolis that put similar Human cities to shame with its sheer scale.
Of course, while many Greater Demons fell in line with Mundus and the council, some rejected such an established government, seeing it as a betrayal of their older, wilder ways. Others fought with the noble families for control of resources and territory, setting off bloody conflicts throughout demonic society. Mundus and the council responded to these threats to their authority by forming their own armies of Demon warriors and sending them out into the world to enforce their will. As Greater Demons who found glory upon the battlefield were rewarded with influence and power by the ruling council and acclaim by society as a whole, the noble families rushed to sign their members up for military service, as the one's success would bring fame and wealth to the entire family. Eventually, the Army of the Demon Emperor was officially established, and it became widely accepted that joining the Army was the fastest route up the social ladder.
As the wars fought between the Emperor's Army and warlords who opposed his rule carried on, a new rank was created for those warriors who had established themselves as true champions of the Demon World, who had truly set themselves apart from all their rivals and would now be known as Devil Knights. These Devil Knights had mastered several forms of combat including swordsmanship, magic, leadership and tactics, and thus were awarded field command over large portions of the Army. The first group of warriors to achieve the rank of Devil Knight included the enigmatic hunter Koshchei, the imposing nobleman Duke Vahagn and the seemingly-invincible swordsman Sparda.
Despite his new prominence in the Demon World, Mundus had not forgotten his initial desire to bring the Humans and the endless possibilities they held under his control. He knew he would need the strength of his demonic brethren to accomplish his goals, but also that few would be willing to go through the trouble of penetrating the barrier surrounding the Human World and battling the Angels for control. So he started on a small scale that many of his subjects would agree with and work his way up. Human souls experienced a cycle of death, afterlife and rebirth, with Humans who had lived chaotic and selfish lives facing punishment from the Angels when they arrived in the Angel World for their afterlife. Mundus struck a seemingly humble deal with the Angels for possession of the evil, undesirable souls in exchange for carrying out their due punishments himself. At the same time, he showed his Demon constituents how they could utilize these Human souls as a valuable resource, earning their support in the deal with the Angels. Thus, to the Humans, the peaceful and benevolent Angel World became known as Heaven, while the violent and terrifying Demon World became known as Hell.
As the followers of the Angels spread their word of the appeals of Heaven and the torments of Hell, some among the Humans found the demonic ways appealing, and continued to follow them despite the Angels labeling such demonic behavior as undesirable Sins. Little did they realize that they were playing right into Mundus' hands; however small his current influence over the Humans was, it was slowly growing, and he had gotten his foot in the door.
Demand for Human souls and Human bodies grew within the Demon World, with many Lessers and even some Greaters developing a taste for Human flesh, and many attempts were made to infiltrate the Human World. It was quickly discovered that the dimensional protections the Angels had placed on the Human World were substantial; only Lessers could slip through the few cracks within the barriers, with the power held by the Greater Demons requiring a magical gateway of proportionate strength to allow them passage.
Finally, after hundreds of years of development, of demonic influence growing in the Human World, of the slow decline of the complacent Angels, Mundus knew that the time was ripe to bring his plans to fruition. Appealing to the desires of the Demon World council, Mundus presented his grand proposal: invasion and conquest of the Human World. The Human World had once belonged to them, he argued, and was theirs by right. The Humans, who were seen as weak under the guidance of Angels, did not deserve their paradise. If they could not defend their world, they did not deserve to remain in control of it. The Demon council and the citizens under them, Greaters and Lessers alike, responded with overwhelming enthusiasm; this would be the greatest undertaking they would ever attempt.
As Hell's armies prepared themselves for the attack, Mundus set about providing them with their route to the battlefield. Lesser Demons who had infiltrated the Human World and corrupt Humans who worshipped the Demons built several gateways between the two worlds, the most prominent among them being the massive tower Temen-ni-guru, which was designed to be Mundus' palace in the Human World. Once all the preparations were complete, approximately two thousand years before modern times, the invasion of the Human World commenced. The Armies of the Demon Emperor emerged from gateways all over the world, catching the Human nations off guard with the speed of their offensive.
The resulting war was a brutal and bloody affair. The Humans, despite their valiant efforts and ingenuity, were simply outmatched by the fearsome powers of Hell. Despite stiff resistance, the conquest of the Human World was expected to be completed smoothly and efficiently within a matter of months. These plans, however, were thrown completely out the window when Sparda, mightiest of the Devil Knights leading the invasion, suddenly rebelled and stood against the Emperor's Army with unprecedented strength. Trouncing the stunned demonic forces, he fought his way through the Temen-ni-guru and faced Mundus himself in single combat. After a titanic battle that shook the tower's very foundations, Sparda achieved the impossible and defeated his Emperor, thought by many to be a god. Though he had won the battle, Sparda found that he lacked the power to kill the Emperor outright, and so instead sealed Mundus into a pocket dimension, using his personal weapon the Force Edge to hold the seal in place.
Knowing that simply defeating the Emperor would not stop any Demons who still wanted to carry on with the invasion, Sparda crafted a ritual using powerful ancient magics that reinforced the barriers separating the worlds by using the blood of those on either side; his own Demon blood and that of a Human priestess. With the main gateway at Temen-ni-guru sealed, Sparda set about sealing those that remained around the Human World, then spent several hundred years quietly watching over the Humans.
Meanwhile, the Demon World was thrown into turmoil. With the loss of their Emperor, the flattening of their armies and the defection or disappearance of their champions, the demonic society was left at a loss as to how they should proceed. Several noble families saw the chaos as an opportunity to steal some power for themselves, and warlords that had been laying low resumed their activities with a vengeance. With the carefully-constructed unity crumbling around them, the Demon World council scrambled desperately to hold their society together until finally Duke Vahagn reemerged and established martial law with the few units still under his command. Working with the council to regain the loyalty of the rebelling noble families, he fought to rebuild the Demon World's armies and the authority that they enforced. The reconstruction proceeded slowly, with Lessers quickly replenishing their ranks but suffering from a shortage of Greaters trained to take up command as officers.
By the modern era, the Demon World had regained some semblance of its former self at the time before the invasion of the Human World. Their reconstruction stunted by infighting and social strife, the council was still unable to select a proper replacement for Mundus as Emperor, and finally began their search for a suitable candidate. Duke Vahagn, thus far focused entirely on rebuilding his armies, opposed the move, instead determined to find and free what he saw as their one true Emperor from his prison. When an agreement could not be reached between the Duke and the council, who saw Mundus' single-minded desire to conquer the Human World as having brought disastrous results, Vahagn left the council's rule and struck out on his own with a significant portion of the partially-rebuilt Army, many of who were warriors loyal either to Vahagn or Mundus over the council.