The Shattering was a major event immediately preceding-and ending-the Core War.

Following the final battles of the War, the planet of Spherus Magna started to become more and more unstable, and soon, the various battles took their toll on the land.  The energized protodermis beneath the surface began to well to the surface, and split the planet at every point it sprouted from, severing the planet into three pieces. 

The jungle planet of Bota Magna, the water planet of Aqua Magna, and the desert planet, the largest, Bara Magna.

For some time after the actual event, the planet of Bara Magna still shook on an hourly basis.

During the actual event, the planet crumpled in on itself, making misshapen lumps for all three planets. Because of the Shattering the Holy Spherus Magna Empire could take root in the belief that an angry god was watching them, and had caused the Shattering.