Thousands of years ago, an age of prosperity reigned supreme due to the unfailing direction of its long-time leader. But as all things must, it came to an end; the beloved emperor grew old and a fear of the end overtook him. He began an effort to scour the country for forbidden dark arts that would restore his youth or extend his lifespan. Court Magisters began combing through ancient tomes and hidden vaults of knowledge. Eventually, they found what they sought. They were able to craft a ritual that they believed would restore youth and resilience to their beloved emperor while keeping his aged wisdom. After all, it was solely through his guidance that his people were experiencing such an age of prosperity.
So they made the preparations and performed their ritual, and it seemed to have worked. Before the Magisters' eyes, their Emperor grew younger, back to his prime. His back straightened, his skin unwrinkled, liver spots disappeared altogether. His eyes returned to a healthy blue instead of a cloudy gray, his hair reverted to its original blonde, as well. It was as if the last 60 years had never happened.
... But it wasn't exactly because of the ritual. That Magisters, in their desperation to save their Emperor, ignored many of the warnings in those old dusty tomes. They knew nothing of the Ethereal Veil, Effusia, or the Saroff, but they were soon to find out.
When the forbidden ritual was performed, it drew power from a source not natural to this world called Effusia which was spread throughout the void. And for this power to be used, it had to pierce the veil that was protecting the world from the hidden things that lurked out in the dark expanse. When the power they drew from the void pierced the veil, something else managed to slip through along with it. It called itself Alsvartr, and it was a Saroff, a race of ethereal beings that lived in the darkness outside the world. For a while, they were fooled into thinking everything worked as planned, blissfully unaware that their Emperor was long gone.
The times began to darken over the succeeding decade as the emperor became reclusive, confiding only in the Magisters who had summoned 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 again and allow through more of his kind. Over the span of several years, he achieved his goals. The Saroff began seeping into the world like a valve slowly being opened.
Saroff took on a plethora of shapes when they entered the world. Their individual power manifested in many different ways. This is how the dark races, the Ethereal Hordes, came to be. Different monikers were given to the common forms they took such as Orcs, Goblins, and Kobolds. The populace had to fend off these terrible beings by themselves as their Emperor cloistered himself in his fortress. Humans fell by the thousands to the teeth and claws of the Saroff.
However, when a human slew a Saroff, they took on new, fantastic powers by tapping the Effusia it released upon death. With this new power, they gained the ability to bend reality to their will. And the more Saroffs they destroyed, the more powerful they grew. When the Saroffs as a whole realized what was happening, they made it a point to prioritize those humans containing the most Effusia. As humans could tap into the Effusia of fallen Saroff, so too could Saroff retrieve it.
The Magisters assisting the Emperor eventually discovered what had truly happened and, in order to make up for their grand mistake, began to devise a trap. Working in secret, they crafted a ritual that would allow them to banish the Emperor back beyond the Veil. In order to do this, they were going to need a great deal of power. After a time, they began working on bringing their plan to fruition.
They informed the Emperor that they had created a grand ritual that would grant his people wonderful resilience so they would be able to better fight off the Saroff. This worried the Emperor until the Magisters informed them how they wished to do this. They intended to gather as much of the population as possible into the city before they performed the ritual, granting this resilience to thousands simultaneously. The Emperor knew this would draw a massive amount of Effusia through the Veil, damaging it severely and allowing Saroff to flood in almost freely.
When the time came, the citizens gather. The enormous crowd was made up mostly of men, which the Emperor thought was a bit odd. The Magisters consoled him, remarking "Who better to gift with resilience than those that will be fighting the Saroff". This was not untrue, and it appeased the Emperor, so they were able to continue their preparations. All those in attendance had slain a number of Saroff and appropriated their Effusia and, when the signal was given, every last one of them will their Effusia into the Magisters so that they might have a chance of succeeding and banish this monster into the void. Many passed out from exhaustion and the trauma of losing their newly gained power, but it was not in vain. The Magisters finished their ritual and the Effusia of thousands of warriors rushed into their bodies.
The emperor was completely caught off guard. He had no time to react before the Magisters were able to surround him. To the last, they knew what had to be done to banish the Saroff and, to their credit, not one of them hesitated. They sacrificed themselves in a massive explosion of energy that they contained as much as they could, creating a deep well where they stood. But they were successful. Alsvartr was launched into the void and finally banished. In the aftermath, the Magisters became god-like figures and were worshiped as such through cathedrals and statues erected in their honor. The well that was created became a place of pilgrimage for the populace.
In order to prevent something like this from ever happening again, the surviving 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 punishable only by death. These precautions resulted in 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, though that was 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 and forced to vow they would not spread their knowledge under any circumstances, even to their own family, lest it weaken the veil further and the Saroff come back in force.
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 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. The nations combined their forces and created a mighty military force that was able to begin pushing back the hordes.
During one fateful battle, the new Emperor of the humans and Grandmaster of tactical strategy was devastatingly wounded in battle. This sent the humans reeling. They lost composure and beat 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 save him. In old and hidden tombs created by original Effusia users not wanting their arts to go forgotten, they found their answer; a ritual that would heal their beloved Emperor and restore his strength in order to continue fighting off the hordes.
The scholars performed the ritual in desperation, fearing that if they didn't have their commander, they would surely perish against the infinite numbers of the dark hordes. But just like last time, their commander wasn't the same when he was resurrected. Alsvartr returned again 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 while he regained his strength 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. A thousand years is a long time to think on what one could have done differently.
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.