Thousands of years ago, an age of prosperity reigned supreme due to the direction of its long-time leader. But as all things must, it came to an end; the beloved emperor grew old and passed away. The kingdom went into mourning. Court magisters began combing through tomes of forbidden arts and crafted a ritual that they hoped would bring their beloved emperor back from the realm of the dead, something that had never before been attempted.
They performed their ritual, but what came back wasn't the emperor they had known and loved. What they had brought back wasn't even human. It was a powerful Saroff, a race of ethereal beings that lived in the space between worlds, that called itself Alsvartr. For a while, however, they were fooled into thinking otherwise.
The times began to darken over the succeeding decade as the emperor became reclusive, confiding only in the magisters who had resurrected him. He worked with them to craft other rituals he claimed would keep death from finding his subjects, but his true motives were to weaken the veil and allow in more of his kind, the Saroff. Over the span of several years, he slowly achieved his goals. The Saroff began seeping into the world like a valve slowly being opened.
Saroff, when grounded, can take on a variety of forms due to their mutability. This is how the dark races, the Ethereal Hordes, came to be. Different monikers were given to the different common forms such as Ogres, Orcs, and Goblins. The populace had to begin fending off these terrible beings by themselves without the guidance of their Emperor. Humans fell by the thousands to the teeth and claws of the Saroff. A glimmer of hope appeared, however, when the humans who slew Saroff began to take on new fantastic powers, able to bend reality to their will.
The magisters assisting the Emperor realized what was happening and began to devise a trap. Working in secret, they crafted a ritual that would allow them to banish the Emperor. In order to do this, they were going to need a great deal of power, so they sent word out into the populace to funnel their power to them at a designated time in order to rid the world of the Saroff.
When the time came, all those who had gathered Effusia in the fight against the Saroff willed their newfound powers to the magisters in a last-ditch effort to save everything they knew. The magisters performed their ritual and the Emperor, caught off guard, was banished from the world and sent screaming beyond the Veil.
However, the magisters, in their great effort, were unable to save their people without sacrificing themselves. So they did. In the aftermath, the magisters became almost god-like figures and were worshiped as such.
In order to prevent something like this from ever happening again, the new leadership cleansed all traces of the events and all lore on the forbidden rituals from the lands. Practice or suspicion of practicing the forbidden arts was a crime only punishable by death. The outcome of this act was the slow regeneration of the Veil over time. Eventually, the intrusions of the Saroff stopped completely, leaving only the Saroff who were already on-world for the population to deal with, which of itself was still no small problem.
Fearing any possible recurrences, the new leadership began a century-long project of eliminating all knowledge of Effusia and the Saroff beyond the Veil. All who had a hand in fighting off the Saroff were sworn to secrecy, not even permitted to spread the information to their own family.
The Saroff, realizing they were cut off from reinforcements, began organizing themselves to prevent detrimental losses to their numbers. Death for them on-world meant they would be banished beyond the Veil. They became reclusive, settling mostly in the places humanity had forgotten. Myths and bogey-men are what became of the Saroff for many centuries.
In time, however, the Saroff grew restless. They grew bold. They grew careless. Forgetting their origins, they began raiding nearby settlements. When those were exhausted, they ranged even further. It took little time for the humans to take notice and respond. They banded together and created a mighty military force that was able to begin pushing back the hordes.
During one fateful battle, the leader of the humans fell to an enemy blade. This sent the humans reeling. They lost composure and began a retreat back to civilization to recover and reorganize. After hearing news of their commander falling in battle, several scholars of lore began searching for a way to bring their leader back. In old and forgotten tombs, they found their answer; a ritual that would call his soul back to his body.
The scholars performed the ritual in desperation, fearing that if they didn't have their commander, they would surely perish against the brute force of the dark hordes. But again, their commander wasn't the same when he was resurrected. Alsvartr returned with a vengeance.
Alsvartr seized control of the situation immediately, withdrawing the human forces back to the capital and walling in the city under the guise of protecting the citizens when, in reality, he was protecting the Saroff from the humans. Alsvartr began work on a grand ritual that would grievously weaken the Ethereal Veil to allow himself and other Saroff permanent access to the world. In short order, he succeeded this time.
His success brought about an unintended consequence, however. The weakening of the Veil allowed the souls of the most resilient and determined humans to remain with their mortal coil, resisting the pull of the Veil into the realm of the dead. In this way, they were able to return to life and continue the fight against the Saroff for the salvation of humankind. In this way, Alsvartr may have doomed himself and his kind. In this way, your story begins.