Calamities for Lost Civilizations

Well, things are a bit fucking bleak today, eh? Let's talk about the end times.

Between current events in the US of A, my work on my archeological hearth-fantasy setting, and my revisiting of the storyline of the latest Final Fantasy XIV expansion, I've been thinking a lot about dysfunctional, decaying and all-around disastrous empires and their ends.

All of Ben Robbins' West Marches posts are incredibly useful, thought-provoking, and entertaining reading, but my favorite has always been his musings on Layers of History. I'm trying to invoke this especially in the stuff I'm writing lately, and it always helps to have some jumping off points for the Big Foundational Questions of a setting - the character of the ancient people that came before, and the reason they're no longer here. So here's some sparks for the long-gone, half-forgotten kingdoms of your setting. Be forewarned that bad vibes follow.

Ruine Eldena, Caspar Friedrich 1825

Roll d4 to determine the nature of the civilization's downfall, and another d4 for the specific flavor thereof:

1. Gradual Decline
1. Magical researchers serendipitously opened a portal to another plane, a land beautiful and majestic and plentiful in land and resources and opportunity. The powers that be attempted to institute a lottery system to regulate those who would be allowed to emigrate to the other land, but the number of people that snuck, bribed, or forced their way into the portal and a new life quickly grew out of control. The portal was ordered collapsed, resulting in a mad rush by all and sundry to enter before it was too late. By the time the portal was closed, so few remained on this side of it that they had no choice but to leave their derelict home behind.
2. A once wealthy and cohesive society fell victim to rising wealth inequality. Corporatization and aggressive profit-seeking failed to self-correct, and society regressed into a neo-feudal constellation of industrialist-controlled keeps and their resident serfs. Over the centuries, technological developments were forgotten, abandoned, or mysticized into primitive cults of machine-worship.
3. In a turn of conscience that stretches even the credulity of the fantastic, the failing empire gracefully managed its decline, returned its stolen land and resources to their rightful owners, and through mass emigration or integration the society peacefully melted away into the greater mass of humanity (lol, lmao).
4. An obsessive belief in superiority over other societies, a cultural taboo against trade, and an overvaluation of the professional class eventually resulted in a devastating wave of famine. People starved to death in the streets, nobles in their gilt palaces, rather than accept gifts of food, labor, or knowledge from rival nations.
2. Self-Imposed Calamity
1. A biological weapon in development, meant to quash dissent, was stolen by a sect of nihilist cultists and unleashed on the populace. The horrific plague that followed brought the ruling government to its end within a week. Tragically, the suffering common folk took far longer to die. The cult still exists to this day, moderated and reformed into a respectable, widely-observed religion.
2. An ambitious conqueror-emperor led their people on a crusade to annex the lands of several bordering nations. The campaign failed spectacularly, and within months the intended victims were picking through the remains of the homeland. The emperor fled as their people were slaughtered.
3. The well-developed and wealthy nation was founded on a principle of minimization of suffering at all costs. The ruling philosopher-council, true believers all, ultimately determined that despite the great strides made and best efforts of all citizens, the complete eradication of suffering was ultimately impossible. At the height of their power and prosperity, every citizen willingly took the hemlock draught and left this world, leaving the trappings of their society unblemished and abandoned.
4. The people built a difference engine, a grand machine on mountainous scale, complex and clever enough that it could be said to have a mind of its own. They asked it all manner of things, questions large and small alike. But one question, the last question, The Question perturbed the machine so deeply that it could not bear to share the answer. Something clicked, and shifted deep inside the colossal mind, and in an instant the people, the onlookers and every other soul in the land besides, found themselves in a strange place, countless miles away. The engine had removed them from its presence before it could recount the terrible truth to them. And search as they might, they never found their way home.
3. Act of Nature
1. Attempts to increase the fecundity of the soil angered a spirit of the plains, and overnight staple crops spread across every mile of the nation. Cities were drowned in wheat, like golden waving tendrils of a vegetal leviathan. The plants clung to soil and stone and flesh alike, growing unceasingly until they were the only life remaining. Then, once their roots had drained every nutrient they could, they died, fossilizing in place. The nation remains a great stretch of dead, blasted land to this day.
   
2. A sudden, unexplained shift in local gravity caused the civilization's grandest city to fold on itself in an infinite spiral. It's now a cursed blot, a black hole in the land that all give a broad margin. Scavengers poke around the edges for priceless relics, and return wealthy but irrevocably altered. It's theorized that the people of the city still live inside the interminable curves of the spiral, distorted and drained of anything resembling familiar humanity.
3. A sudden climate shift saw the iceberg-dwelling peoples of the great seas losing space in their rapidly melting cities. As they observed a religious dictum never to set foot on solid land, they chose a quick end, rending the ice beneath their feet apart with deafening cracks and capsizing their failing homes. Their relics and stone and driftwood homes still exist in the darkest chasms at the bottom of the sea.
4. The treetop cities of the folk of the wood were glorious to behold, shaped with the skill of carvers and the affection of the trees both. Then a blight came that made the life-giving sun poisonous to the trees, stripping the living columns of their golden leaves and turning their smooth red boles tar-black. Some of the people stayed in their cities until their trees fell, dying with the homes they loved so dearly. Some fled, hearts filled with shame. And some dug deep beneath the earth, and turned their trees topsy-turvy into the yawning caverns, planting them root-side up so they could live on in the darkness beneath. Do they live still?
4. Act of Divinity
1. A corrupt priest, having achieved the gift of immortality through skillful manipulation of the society's religious tenets, was cursed by their deity to become violently destructive to humanity and anything created by their hands. The priest founded a cult around their strange properties, and was worshipped adoringly as civilization crumbled around them. They danced into oblivion until nothing was left but the rubble beneath their feet. 
2. A guardian deity, enormous and anthropomorphic and ever-present, gradually lost the respect of their flock; as they encountered other peoples whose gods were less hands-on, less demanding, they grew to resent their own. Rather than allow their power to decline along with their worship, the deity chose to annihilate their chosen people with one sweep of their almighty arm, inadvertently eliminating themselves along with their believers - with nobody left to believe in them, how could they possibly exist?
3. The Rapture occurred far earlier than expected, though otherwise just as predicted by the seers. The vast majority of the population ascended into a glorious afterlife, the shining, golden gates of which can still be seen in the sky above the abandoned capitol to this day. The few sinners left behind abandoned their former homes in despair, and their cursed descendants still wander the land in nomadic family units.
4. The grand city-state atop the cliffside was beloved by its god, a passionate and impetuous deity, youthful as gods go. When the cliffside crumbled, and all its followers began to plunge into the inestimably deep chasm below, the god panicked. With a snap of their fingers, all the people of that city sprouted feathers, and in a moment they were people no longer, but common swallows. The city now clings sideways to the face of the cliff, where it tilted but did not fall. Thousands of swallows infest every hollow in every building on its face. Their god sits resolute on their sideways throne, deep in thought, stirring once every century or so to attempt a new spell to restore their people to their true forms.