I always imagined something like what happened with the dinosaurs. In Ayenee though the thing that impacted could have been a small planet, the realm was always off. Maybe Ayenee's moon descended down upon the land, whatever hugely powerful beings that existed gave their lives in some noble martyrdom, but only prevented absolute destruction.
Perhaps a comet named Yahell named after the god of creation and destruction descended on the land. Maybe it wasn't just a comet, but some alien ship. Before impact the vehicle's occupants jettisoned and after the intense nuclear or whatever energy blast. The survivors of the world were forced into battle with an alien entity and though the Ayenee citizens won, the world was left in a shabby state.
Maybe all those powerful beings caught the attention of something even more powerful. It came to Ayenee seeking a fight like the rest of the universe had been unable to give it. After the dust cleared the world was left without its protectors and in a state of ruin from the fight that took place.
Maybe some powerful congregation of powerful beings came together to perform some spell to lock out some danger to Ayenee. They may have tampered with powers beyond their combined limitations and brought destruction on themselves and the world.
Or perhaps the constant powers that were always in use within Ayenee were its demise. The constant time travel, plane shifting, massive power sinks leaving magical sinkholes behind. Maybe the lands is being torn apart from the abuse of abilities and magic by its citizens.
Yeah I didn't add question marks, so what?
Maybe it was something like a magical/ technological apocalypse between the two poles of Ayenee. One pole being the one that allows magic the other being that which cancels magic. The two sides brought tech and magic against one another, next thing you know there are natural disasters, nukes, spirit balls, skynets, satellites with laser beams, miniature black holes, dark matter ripped from its mooring, leylines rerouted and destroyed, reality fractured, time tampered with. The war to end all wars.