Here are some ideas on what are the results of a cataclysm striking a fantasy setting. These are in addition to the typical results found in science fiction settings, such as desert wastelands, ruined cities and roving gangs. 

Forests of strange plants center around altars of Chaos. Every year, season ,week new creatures emerge to harass or slay people who live near the forests. These creatures are unique and sterile, so no one knows their weaknesses.

Seashores covered in trash that includes idols of lost gods, busted wands and staves, corpses of merpeople, shattered remains of ships from other eras dragged through time and jellyfish mutating into something else.

Battlefields covered in golem parts haunted by the spirits that animated them. The spirits can possess armor and have captured several groups exploring the area, killing all of them by dehydration. These sets of armor carrying corpses are a source of disease.

Ruined cities where vegetation creeps right up to the edge, but nothing green dwells within. Plant people who enter feel extremely ill and will die if they do not leave within a day.

Floating islands that, if dug into, turn out to be the bodies of daikaiju or gods. Both were worshipped before the cataclysm and no one knew where they went until the PCs make their discovery.

Round hills, the tops of asteroids that impacted in a manner that kept them intact. They are populated by transplanar refugees escaping from dying worlds (Thanks to Clark Ashton Smith for the idea.)

Temples that are swarmed with people, human and other, wanting to know why the gods allowed civilization to fall and what they are going to do about it. Whole tent cities pop up around the major temples and that draws predators, human and monstrous.

Mountains that are the remains of petrified giants (the really big ones). People live in the caves that resulted from open mouths. The intestines and skin are swarming with monsters, former parasites that survived the death of their hosts.

Looking at old star maps, one discovers that they differ now. It may be the constellations have been altered by a divine war, the stars that where homes to the losing gods have been snuffed out or somehow time has been warped so that those stars visible during the winter use to be the stars of summer.

The sky shows signs of scorching. It is a different color or has patterns that didn't exist before. If there was a divine battle, only the area of the sky where the battle occurred, or was above, show this change.

Magic has changed and for the worse. Simple rituals are much more difficult and spells that use to be the providence of archmages now are either impossible or automatically attract the attention of demons of the apocalypse. People lament this loss in song and art.

There are pieces of art of extinct people, animals, crops or monsters. Such works cause melancholy in all who view them (this is a supernatural effect).

Mechanics acts as tinkers that repair people instead of objects. Their arcanotech does take away some of the damage, mental and physical, that the cataclysm caused to the people, even these many generations hence. What the mechanics do with this damage, no one knows.

Lakes that were created by orbital impacts when the gods' throne moons (or moonlets) came crashing down. They are haunted by the ghosts of servitors, psychically scarred angels or similar beings.

If a ruler or someone in their bloodline either started the cataclysm or took an important part in the ruin of civilization, all coins with their face are either partially melted or their appearance is warped. If the bloodline is cursed with lycanthropy or something similar, the coins can be used as sling stones to harm them no matter what metal they are made of.

Races' lifespans have been reduced. Elves that use to live to see 5000 years now die of old age at 200. Even dragons see the end of their days within a matter of decades or centuries.

Hyperactive weeds prevent all agriculture. Every time crops are planted, the weeds appear and smother them. Attempts to use magic to stop this makes the situation worse as the plants then dig themselves up and attack people.

People and locations have retreated to the Realm of Dreams.

Mirrors show the world as it should be had the cataclysm not occurred.

The doors to Hell have closed and no one can contact the devils. Any liens on souls have been revoked and the powers or riches gained vanish.

Blind monsters, former oracles, wander the lands and consume all they encounter. They mutter partial prophecies they gave to others before the cataclysm struck.

Shadows detach from the dead and form a shadowy city or forest. Those who visit such a place come back different, able to see into the spirit world when drunk or high.

Unity has failed among the insects. There are no more swarms of any sort. Those creatures that specialized in preying on them have gone to alternate dietary sources.

Harmony is impossible in the human mind. Classes that depend on inner tranquility no longer exist, at least in the human population.

Individuality is no longer possible in one of the non-human races. They either all act alike or have a hive mind mentality.

The definition of beauty has changed so much that art (portraits and abstract) from the old world disgusts all who view it.

In the wastelands that formed during the cataclysm, women can not become pregnant. This sterility is not permanent and does not occur elsewhere.

All pets and livestock have gone feral and nothing can tame them. Crops may also go feral- that means they pull themselves up and start eating people.

Termites and ants have become gigantic and war over the last few green places. Unlike in the real world, the termite armies are equal to the ants’ and so there is equilibrium. If someone was to disrupt that, the result is a carnivorous monster or herbivorous monster gone wild.

Children’s play has become more viscous, related to survival by defeating anyone that gets in their way. This applies to games such as checkers as well as roughhousing. And the adults that they turn into are better at surviving the state of the world, though at a cost of social thinking and behavior.

Rain, lightning and other weather phenomenon slows greatly, to the point where an observant character can dodge natural lightning most of the time.

Demiplanes appear that merge locations on two planes, allowing movement between them. The actual demiplanes are not parks; but rather hellfire pits, radiation tortured landscapes or darkness infused ocean floors; so few creatures actually attempt the trip.

Trees ooze dark sap that can be dried and turned into cloth. If someone were to view the stars through this cloth, they would see the prison where the gods have been locked away.

Doppelgangers exist for everyone, the result of all of humanity splitting into two after birth. If one is slain so is the other. They are not good and evil, but rather spiritual and material. Doppelgangers can not stand the sight of their other half and do anything to escape such an encounter. If they come into contact, they fuse and become something monstrous.

Gigantic crystal caterpillars are consuming the moon(s) and the lunar god’s pain is felt by everyone in the world. This pain is driving everyone to desperation and foolish actions are much more common than before the coming of the insects.

Ogres were exposed to psionic energy and are becoming more and more intelligent with every generation. They may very well ascend to an energy form in the near future and then take their ancestral frustrations out on the world.

The seas have become toxic and rough, so much so that ships are a thing of the past. Every so often something pulls itself onto the shore, a jellyfish/clam/whale thing that tries to find a lake to make its new home. If that happens, the lake is likewise corrupted until the beast (and possibly its spawn) are slain and burned.

Forests are retreating into the depths of the world. The trees themselves are so terrified of whatever caused the cataclysm, they are forcing themselves to adapt to a life in darkness.