What people think it was
Most people believe the Cataclysm was a single event that destroyed the world as it was and remade the world into the way it is now. They believe this took no more than a day, though details vary and it features in every culture and every religion. The way it fits into each culture and theology however is different.
The Church (in Valencia) states the people of the seven races were morally corrupt and so god destroyed everything they knew and gave civilization a second chance to get things right. Before the advent of the Church in Valencia the Cataclysm was often called ‘the rebirth’ and was seen in a fairly favorable light caused by the request of the gods. This belief however has died out over time.
What it really was
The real Cataclysm wasn’t just the enemy bombardment from space or the invasion forces that landed, but a period of at least 2 generations afterwards as people lost knowledge and skills used in the past. In stead they invested in new skills needed just to survival in the changed world. This is the key turning point for the world of Celestia since original colonization. It is also the single event reshaping society for the humanoid races of the world.
It left thousands of artifacts spread around the world, as well as ruins for all buildings that were not outright destroyed. It also created several deserts around the world due to bombardment by energy weapons.