Great Cataclysm

The Great Cataclysm is known as the most severe catastrophe in Carmia's history. It's responsible for the fall of the United Imperium of Carmia, an enormous death toll and still lingering phenomena like Mormo's Corruption.

Initial event
A reality rippling surge of supernatural energy erupted from the capital city and engulfed the majority of Carmia's mainland. Records tell about the sky becoming so bright that those who saw it got blinded, some of them loosing their eyesight permanently. Nearly all survivors recounted loosing consciousness, only awaking after at least a day must have passed and finding themselves in completely different locations than before. Roughly a tenth of Carmia's population died instantly through unknown causes. Those who woke up again experienced high magnitude earthquakes, hazardous magical forces, fluctuations in the weave of reality and Mormo's Corruption during the following weeks.

Aftermath

 * The land was reshaped. Coastlines changed. New islands were formed. Large cities and landmarks were swallowed by sinkholes.
 * Numerous of Mormo's monstrous creations were unleashed upon the world and wreck havoc to this day.
 * Countless survivors have been exposed to the Corruption while being wounded, leaving lots of them with twisted bodies and minds. Some of those might be the origin of several monsters.
 * Several species native to Carmia died out.
 * A large number of rifts to other planes were formed, spiriting many of Carmia's inhabitants away and allowing outsiders like Fiends or Fey to freely enter the Prime plane. Some of those rifts remained permanently open.
 * A number of Chrysalis vanished without a trace.
 * The Scar was created. And it may never heal.
 * With the destruction of the capital, the disappearance of Mormo, the deaths of many influential figures and the general turmoil, the United Imperium fell apart. The survivors formed numerous tribes from which new nations arose later on.
 * Many of Mormo's secrets could now be unearthed from ruins, making advanced knowledge and technology wider available. This led to a sprawling culture of delvers and their discoveries would brings lots of progress and change.

Explanations
While there exist all kinds of different theories, most of them have one aspect in common: The Cataclysm must have been the result of one of Mormo's experiments going awry.