The Sundering was a great magical calamity that occurred 1200 years ago, centered on the homeland of the great Human empire of Xandala. At the time, the Human Arch-Mages of Xandala were perhaps the most powerful in the world, and their influence was felt in places as far away as Uqebesh and Obeka.
Cause
Not much is known for certain about the cause of the great catastrophe. Human legends tend to focus on the great Mages of Xandala, particularly the Wizard Emperor Usuvior. In most of these stories, Usuvior in concert with the other Mages attempt to gain forbidden knowledge or power, and are punished by the Gods as a result.
The oldest Elven sages are potentially only two or three generations removed from the event. However, they have not shared any insight they have of the event with outsiders.
Effects
It is known that there was a magical explosion that completely leveled the capital city of Xandala. In addition, a great magic surge emanated outwards, causing huge amounts of land in North-Western Eao to sink underwater. This also led to the formation of most of the islands in the Ten Thousand. It has been speculated that the newly formed island chain is what has allowed explorers from distant Xiqui to reach the known world. However, it has also been suggested that the pre-Sundering empires had more contact with the Xiqui, so the truth of the matter is not known.
A great Earthquake rippled through the entire continent of Eaoder, causing untold destruction to the Humans, Dracon, and Elves that lived there. Most especially devastated were the Dwarven and Rodian peoples. A massive tidal wave struck the Northern shore of Ogrua, causing great devastation there. The Kimer people living on the edges of the Koswana Bay were least affected - having been sheltered from the tidal waves by The Chain.
Aftermath
Much of the world was changed after the Sundering. The Elves began their withdraw back to their homeland of Ogrua. The Dwarves and Rodians, reeling from the worst losses, were finally able to put aside their differences in a truce, ending the Cleansing War that had raged for thousands of years. Already relatively few in number, the Dracon people entered a decline that continues to this day. Kimer living to the West of The Chain, the great land bridge, were devastated and their numbers have never recovered. In contrast, Kimer living to the East of The Chain were relatively unaffected.
While the Human empire of Xandala was destroyed and most of its people killed, its destruction (as well as the losses felt by Elves, Dwarves, and Dracon) created a vacuum that new human empires and city states have filled. The great cities of Isulias, Ajanta, and Vok River all rose to prominence in this time.