The Sundering, also known as the Sundering of Toril and Abeir, can refer to two different—but similar—great realms-shaking events in the history of the worlds of Abeir and Toril. The first Sundering of the worlds happened many millennia ago, while the second took place during the decade of the 1480s DR. Most commonly, “The Sundering” refers to this Second Sundering, as it is what people have experienced in their lifetime.
In the first ages of the world of Abeir-Toril, the mighty primordials, born of the elements, battled the gods for control of all that was. Before the world was completely destroyed by these battles, Lord Ao intervened and sundered Abeir-Toril into two twin worlds: Abeir and Toril, giving the former to the primordials and the later (the original world) to the gods, ending the conflict. Both worlds existed in the same Prime Material Plane, but occupied different dimensions.
When Ao the Overgod destroyed the Tablets of Fate at the conclusion of the Time of Troubles, he instigated the Era of Upheaval. The Tablets defined the laws of existence and kept it relatively stable. Without them, chaos ensued and the worlds of Abeir and Toril, separated many thousands of years ago, slowly started to overlap. The Spellplague drastically sped up this process.
The Second Sundering started in 1482 DR with Ao’s decision to recreate the Tablets of Fate, and to separate the worlds of Abeir and Toril once more. Complete by 1489 DR, the “status quo” has returned the worlds and gods to much the same as it was in 1357 DR, with some differences, of course.
