Castoria was once a thriving and prosperous nation, a rich trade center for the surrounding lands. This all changed when, on one fateful night nearly a century ago, the Mist of Eternity rolled in and surrounded the land, obscuring more of the outside world as days and nights passed.
By the time the Mist blocked out the sun, a new light shone during what was assumed to be daytime: The Starpoint Spire, a mysterious place atop Castoria's highest peak in the northern-central region. Some say that there is some sort of building atop the mountain shining the dim "sunlight" onto the land, but it is only ever too bright or too dark to fully make out any structure, not to mention the mountain's immense height.
Not a month after the Spire's light lit up, the stars fell. Flaming rocks and debris from far-flung edges of space plummeted downward onto the eastern region of Castoria. Once the shower subsided, a strange energy from within the fallen stars transformed the eastern lands in what are now known as The Voidwastes, a barren gray land littered with craters and strange alien creatures (these can vary, but I had Pathfinder's Akatas in mind).
To the south, strange mechanism of eldritch origin are again at work after aeons of rest in the Ruins of Kchuthngnl, an ancient city of non-human creation that is estimated by scholars to be no less that five millennia old.
To the west, the once peaceful and serene forest, now known as The Plagued Woods, has been experiencing corruptions of the wildlife and humanoids living there. Some humans have reported creatures that appear not unlike a halfling, except that they can open their mouths to massive proportions to swallow creatures the size of an ogre.
When adventurers and citizens alike try to make an escape from Castoria, they are never seen again, and it is utterly unknown whether they found hopeful sanctuary or agonizing death withing the Mist's depths.
What is unknown to all residents of Castoria is that all of these events occurred because of the actions of a secret but powerful cult loyal to the Elder Gods who call the space between the planes their abhorrent home. The cult still lives on, larger than ever, and their plan is for the alien horrors to incubate and thrive within the dome of mist that now envelops Castoria, so that when the Elder Gods return as the cult's prophecy foretells, they will have an army of blasphemous creation at their disposal that they will use to make war with and enslave the denizens of the Material Plane.