It came without warning.
For as long as the oldest adventurers could remember, the Great Desert was there. Immense and unmoving, it's expanse separated civilization from wilderness. Mortals from magic. Ancient ways of thinking from the newest technology. At least, that's the the adventurers said around campfires in the dead of night. They were the only ones capable enough to cross the sweltering to sands and to see just what lay on the other side. Few returned. Those that did, and in good health enough, spoke of established civilization and grand cities. They had names of grandeur, like Icaria, and Mistral. The adventurers spoke of Templars and great evils, and how utterly backwards the land beyond the sands was compared to their own. How... plain it was. No magic, no advanced technology. Any sort of machinery beyond the basic furnace or redstone was a wonder to them, a great mystery. These people had no generators, no machines to mine for them- no quarries, no computers. Their magic was rudimentary at best, waving a stick around a hoping for the best. They knew nothing of the laws of equivalent exchange, had no idea about transmutation or diving. The beasts in this land were much less terrifying, night or day, and the fauna was only of the most fundamental trees and flowers. It was quite sad, really.
Those adventurers also spoke of other things. Something else that made this land seem almost otherworldly.
They spoke of a wall.
A wall that kept the Great Desert out. The locals spoke of it, the desert, in fear. How odd, as they treated its sands as a hostile entity that had to be held back, lest it consume all in its path. The adventurers laughed, for there was no wall on their side, and the sands still held motionless as they always had. How queer these people were, with their 'skylords' and poor attempts at magic, people who should not hold the title 'wizard'. How curious was their fear, how curious was their superstitions and belief in a single benevolent god. How weird were their, well, their entire lives. The adventurers still only laughed as they headed back across the desert, ignoring the locals' halting cries of surprise.
Stories of this 'land beyond the desert' spread from community, from adventurer to adventurer, from trader to trader. It spread to a magic castle of black rock, to a mad scientist's keep, to a white marble factory where all chuckled at the far-fetched tale; save a solemn spaceman and his dwarven companion. It spread to isolated houses and dirt huts across the wilderness, and all who occupied the land thought how queer, how odd these people were!
And then the sands began to spread. Not all at once in a windswept storm, no- it spread like a sentient being. Slowly at first, meter by meter, mile by mile. But it picked up speed, devouring chunks of the land to replace with suffocating layers of sand. The desert was seemingly alive, digging tendrils of choking grit into everything, anything within reach. The other people, beyond the wall, had been warning those who visited. They knew. They knew the true nature of the beast, they knew the underlying danger. The trees, they shriveled and turned to sandstone by some force even the most accomplished of mages could not name. Animals were found dead with stone bones, bleeding out crimson sand onto the desert beneath. Soon, the adventurers who had laughed at the desert succumbed to it as well.
Most dwellers of the wilderness had no choice but to flee the homes they had built themselves, in a feeble attempt to outrun the desert that had betrayed them. hey did not understand that the sands could not, would not stop consuming until all was barren dust. However, others stayed. Those who managed to carve out a living there were stubborn, like the land around them. To those who stood still and faced the threat, the sand was anything but merciful. Their homes were buried, suffocating the inhabitants or crushing them beneath tons of desert- that is, if the mysterious plague the sands carried did not claim them first. Not to say all died. Some were lucky- or unlucky, if you considered they were trapped in a lifeless expanse that was slowly killing them as they breathed. These survivors, of those who did not take the hint and follow the ones who ran, were forced to become pathetic scavengers. Scratching a meager existence out of the land, day to bleak day; but they had their reasons for staying after all they had endured. Stubbornness. The fact that there was nothing else left, and running was futile. A few stayed because they had lost those close in the panic and sudden arrival of the sands. They scoured the wastes in search of their family, their friends- anyone.
Anyone at all.