Great Cataclysm

The Great Cataclysm was a series of catastrophic events around the Sarpic Seas caused by the explosive eruption of the volcanic island of Alborak 1255 years before the present day (Year Zero in the Vathanalu Reckoning, which uses this disaster as the starting point for its numbering of years). It is sometimes called the "Second Cataclysm", in reference of the terrible events at the end of the Schism Wars much longer ago (which would thus be considered the "First Cataclysm"), but this is not common. 
The eruption completely destroyed Alborak and the city of Qoss upon it, triggered a series of powerful earthquakes up to a thousand miles away, and sent gigantic tsunamis sweeping across the seas to devastate distant shores. It ended Qoss's previously unshakeable dominance of the Sarpic Seas in dramatic fashion, causing the immediate collapse of its empire and annihilating most of its major coastal outposts and colonies around the region. The great cloud of ash and dust thrown up by the eruption darkened the skies all over Rhydain and eastern Orithea for over a year, bringing cold weather in summertime, crop failures, and famine. 
Together, these events ushered in a period of general chaos and upheaval all around the Sarpic Seas and beyond. War, anarchy, and the mass migration of whole nations ensued. The power vacuum left by the destruction of Qoss eventually paved the way for the rise of new powers in the Sarpic Seas, such as the Phyridian Empire, the Asgali Empire, and the major pre-Kirdan Uremite kingdoms to the north.