Towering magic war golems and arcane bombs leave the world an unstable wasteland. Civilization clings to life in small oasis’s, hidden valleys of green, and walled cities. The wastelands between settlements are savage deserts, warped petrified forests, and poisoned lakes. Strange creatures thrive and mutate in the residual magics that linger in the destruction, making the wilds a dangerous place filled with alien horrors.
The moon hatches. As shards of lunar rock crash down all over the material plane, a beast falls into the ocean. Tidal waves destroy coastal cities, and soon after the gigantic creature (potentially larger than a city) carries that destruction inland. A threat similar to godzilla (but even bigger) threatens all life on the material plane.
The god of death is no more, and everyone forgets how to die. Instead of death, mortals linger on the material plane because there is no god to show them the way to the after life. Those who’s bodies haven’t sustained significant trauma remain as sentient undead in their decaying bodies. Over time they deteriorate and become mindless zombies. Those whose death leave an uninhabitable body become ghosts and wraiths. Undeath is everywhere, and all eventually turn on the mortals that still survive. An undead apocalypse ensues.
Magic fails. Magic users become few and far between, and advanced magical societies collapse as arcane infrastructures, such as floating cities, magically directed water streams, and other reliance on magic and magical items from day to day cease to function. Magic items can only be created by finding and breaking down old magic relics to use the magical dust Residuum. (can be more devastating if the world was previously a magicpunk setting where large magical cities populate the map)
The earth crumbles, and soon nothing is left of the world except for floating islands and small continents. Only a fraction of the surface is left and those that survive must adapt to a life in the sky over a bottomless abyss.
The good gods win, and the 9 hells and other evil planes are torn asunder…only to have their remains collide with the material world. Now the home of humans, elves, and dwarves resembles that of heavy metal cover art. Though the dark gods have perished, their armies now enslave the mortal races of the material plane. Dark cathedrals and fortresses, as well as strange warped structures of torture and blood litter the world. The leaderless demons and devils form kingdoms and tribes, maiming, killing, plundering, and destroying as they please.
The oceans rise until only the land at the highest elevations is above sea level. Now mountain peaks are the only solid ground left. Most that survive now live (and die) on the sea. Mortals scramble to create makeshift boats, flotillas, and floating cities as they battle tsunamis and storms, and the denizens of the ocean that now find the land dwellers as easy pickings on the sea.
The gods have forsaken the mortals. As they retreat, all prayers are left unanswered. Those gifted with divine magical prowess are left empty handed and the presence of clerics and paladins becomes almost unknown. Famine and draught have become the norm. The planet slowly dies until those that still survive are left in barren wastes held together by their will (or a powerful leader’s will) to live.
The earth cracked, and beneath our feet they swarmed over us. Some force from the underdark has dug tunnels under much of the surface dweller’s kingdoms, and begin waging war by collapse cities from underneath and surfacing in the center of unprepared populaces. This threat could be something like drow, or perhaps in this world orc and goblins lived solely underground until they’re ready to wage war on the surface world. An example of this are the Locus from the Gears of War series.
A mighty, magical empire rises as a dictator too powerful to challenge claims city after city, kingdom after kingdom. This villain rules with absolute power, and his soldiers are all magically manipulated to lose their sympathy and swear loyalty to this dark emperor. Bonus points if the emperor is a buff, skeleton faced lich with a group of quirky lead henchmen.
The sun explodes, plunging the world into darkness. As the Evernight begins, the creatures of the night reign. Vampire and werewolf populations explode in number, and the citizens of the underdark surface into the now-safe darkness of the above world. Forests die and are replaced with dark, twisted growths from the underdark. Enclaves of holy light become hidden settlements for humans, elves, dwarves, and the other creatures of the light, forever under assault from the oppressing darkness.
Aliens have landed. In sleek, undulating spacecraft, the Illithid have begun their assault on earth as they conquer their way along the stars. Their advanced magic-tech ships ravage the land, destroying country capitals and military fortresses. The mortals of the material plane are either eaten or enslaved until only secret havens remain to live in hiding or attempt guerrilla strikes in vain acts of rebellion.