The Evil Empire Won
As long as we've used stories as moral plays, most stories which feature oppressive cultures or empires would somehow also feature the moment where that empire fell. Some scrappy and outgunned revolutionaries rise up and, noble in intent and resigned to self-sacrifice, defiantly stand against the imperial oppression. Generally, they win out in the end.
For Oshanal, this was similar to how the last age played out, but here the evil empire didn't fall. On a night of a planar conjunction, the maniacal sorcerer-king began his great ceremony, and although the heroes did arrive, they were too late. The ceremony could not be stopped, and the gates to other worlds opened, giving birth to a cataclysm that ruined not just the old world, but nearly the cosmos and eternity itself.
Oshanal mostly survived this terrible cataclysm of eternities, and they were devastating – yet unfortunately, for many, the empire then went on to thrive in the wake of the destruction they'd wrought. In fact, it consolidated their power as they were instrumental in the creation of the Hanging Towers themselves. They even went on to enjoy a tremendous amount of appreciation and good will from the survivors as, for the refugees of the sky world, the imperial armies were the last, most reliable allies in the war between mortal, and monstrous god.
In any event, they were, and are here, still, referred to as the "evil empire" for a reason. Sure, you could say that they fought for mortal life, and that would be true. Or, you could say that they fought for only those mortal lives they thought were worth fighting for, and that would also be true. In practice, there were entire cultures and peoples that the imperials regarded as "less than". These people were refused passage onto the first of the towers, and were left behind to watch it rise into the skies as the floods of Ereshkanoi swept away the world around them.
Today, this empire is in decline, succumbing to the natural entropy and self-loathing of fascist regimes. The Hanging Towers have largely fragmented into independently powerful collectives of Tower States, each claiming domain in their own network of interdependent towers. There is passage and trade, and no one is at open war, but the central lines of the old sorcerer-king have ended, and many new dynasties have emerged in the increasingly cosmopolite world of the sky people.
Meanwhile, below, the surface folk have endured and eventually prosspered as well. The old people of the last continent, and the seafaring folk of the crystalline archipelagos, abandoned long ago, have grown and adapted in unison with the behemoths of the monstrous world. To these people, monsters and titans are commonplace, life goes on, and everyone has adapted and become strange enough over time to perfectly fit their strange twilight world.
Yet, the animosity between the people because of the evil empire burns bright. This empire, once called the Eruvenian Empire, now referred to as the Epicentral Empire, is still a persistent threat, waiting for another "pure-blooded" (such a thing no longer exists) demagogue to rise up and galvanize the dynasties into lockstep again. No one has truly forgotten the cruelty of the old Eruvenians, and many remember that it was an Eruvenian who shattered eternity. Bitter feuds persist between the surface and sky folk, and probably always will.