The Cataclysm is a devastating event that occurred 50 years before the main events of Pandora's Tower. It caused earthquakes, storms, and total destruction. It was worse in Elyria than any other country, and it was the event which created The Scar.
The Cataclysm was caused by the Elyrian Army when they were carrying out the "Final Experiment". After successfully creating 12 Masters with the help of the Vestra, they believed that they understood how the Masters had been created, and decided to create the Final Master without the help of the Vestra. This resulted in The Scar being created and thousands of the men in the Army killed.