The Sundering was the catastrophe in which the gods destroyed the civilization of the alatharya and broke magic into six spheres. It took place early in the history of the Prime Material, and its date is unknown.
Before the Sundering
The gods made the alatharya, the First Children, and gave them magic in its original, unified form. The alatharya built an empire that spanned the world. Their mages helped the titan brothers Ealim and Aomin raise Castle Nathli over more than a century, and with a weapon that drew on the sun they destroyed the god Eloth, whose remains were later laid in the Grave of Storms.
At last the alatharya sought to steal the power of the gods for themselves. Thethtil foresaw the doom his people were bringing on themselves, and built his tomb.
The Sundering
The gods answered with their wrath. They destroyed alatharyan civilization and split magic into water, spirit, air, fire, void and earth. No spell since can involve more than two spheres, and access to some spheres depends on the resonance of the practitioner's soul. Artifacts made before the Sundering still function, but the constraints it laid on casting and creation have never been lifted. Jolinn and the titans fell into a slumber that lasted until the War of Fire.
The alatharya were stripped of their peerless minds, their capacity for magic and their ability to align with a resonance. Their bodies endured, and with Enirra's intervention they survived the chaotic years that followed. Storms of magic swept the surface in the Sundering's wake and changed what they touched.
After the Sundering
Thousands of years passed before civilization began anew. Flightless proto-aelin settled the Daphoa island chain, and around 2750 BCC Linphori the Fearless led an expedition to the highest peaks, where Alia gave the aelin their wings. The Seven Sentinels claimed the floating disc of Ilodaiya and founded Imperial Daphoa, and the aelin gathered what artifacts of the alatharya they could find.
The human legend of the Sundering is told on the World History page, in the human telling of the ages.