The Sundering is a reoccuring earthquake of catastrophic proportions that takes place once every 200 years.
It happens when The Drom Fault, a fault line that runs from The Drom Canyons down throughout all of western Ulia, ruptures. This rupture happens every 200 years without fail at an extremely precise time.
The reason why such a catastrophic event has gone mostly unknown for all recorded time, is because outside of The Sarai, no other nation was ever affected by it.
The Sarai's Pharaoh, who has been the Earth Elemental for thousands of years, was given the divine task by the Gods to be the one to stop the earthquake. Every 200 years, the current Pharaoh would venture across the land to The Droms, and at midnight, when the rupture began, he would use all his strength and power to not only stop the disaster before it could fully release, but return the earth to rest for another 200 years.