The Empire of Tolos was at its height. For a hundred years, it had ruled the world with an iron fist, its might and splendor unmatched throughout history. Blood sacrifices made to please the gods attributed to her rulers magical powers which mortals were never meant to wield, powers with which they could decimate armies or rip apart castles. On the first day of the four hundredth year since the founding of the Empire, Emperor Cyrus II gathered countless slaves to sacrifice in the Imperial Capital of Tolos, in a sick ritual meant to make him a god.
That... Didn't work out.
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The End Times
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It's been a month since that day, known as the Ascension. Whether the Emperor succeeded or not in his mad bid for godhood is unknown, and rather irrelevant, given the horde of demons which appeared out of nowhere that day to lay waste to his empire. There were no survivors to describe the ritual, as the entire capital city of Tolos was ruined apparently instantaneously, killing everyone inside the great city. Hordes of monsters fanned out into the surrounding lands, butchering thousands and laying waste to helpless cities.
A month has passed since that dark day, and things have only gotten darker. The destruction of the capital has thrown the entire empire into chaos, as central authority breaks down. The Legions are acting without any kind of coordination; some are rushing towards the heartland of the Empire to hold back the tide of evil spilling forth, while others have remained in their territories, their ambitious generals taking control as petty warlords. The autonomous kings and princes do much the same. Those far from the capital have mostly declared themselves independent, hoping that others will contain the demons for them, while those near the heartland have suffered catastrophic defeat after catastrophic defeat.
You are one of the great men and women of the world. Perhaps you are one of the few who wield magic, your considerable powers a gift from the dead gods which still roam the earth. Or perhaps you are a mortal ruler, one of the quasi-independent royals who once administrated the land in the name of the Emperor. You might band together with each other to fight the demons, or maybe you'll use the opportunity to right past wrongs.
What's certain is that there is no hope, for you or anyone. The limitless hordes of Demons can be stalled but never defeated. Eventually, every corner of this world will be overrun, every last life snuffed out. This is the apocalypse; the end times.
TL&DR: The apocalypse happens in a fantasy empire, you're overpowered super-mortals who have to try to avoid dying for as long as possible as civilization collapses around you. Good luck.