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 Realmspace 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 Sundering involves the re-separation of those worlds. In addition, Ao will rewrite the Tablets of Fate, and the gods, unsure of what this will do to them and their power, make efforts with their most powerful servants to prepare for that event.
When the Elves carried out their Sundering thousands of years ago, it frayed time in both directions and allowed some fortunate (or cursed few) to glimpse the past and present of Realmspace. One of the mages who witnessed the event had a vision of two other Sunderings, massive in scope and far mightier than what the Elves accomplished. One stretched far into the past, while another waited in the future.
Far back in ancient history, the creator races of Faerun were engaged in great battle that threatened to destroy all of Abeir-Toril. Powerful creatures called Primordials rose, each attempting to conquer the fledgling world for themselves, and the gods met their challenge. The battle became so fierce and the consequences so destructive that it got to the point that the Primordial Asgoroth the World Shaper threw an ice moon at the world, claiming that if he could not rule it, then no one could. This cataclysm caused massive damage to Abeir-Toril and is recorded in history to this day.
At this point, Ao stepped in and worked a great Sundering, the first of its kind. He twinned the world and split it into two: Abeir for the Primordials, and Toril for the gods. Each world would hold onto the vestigial name of the other, but it was primarily a point of sagely academic research. Part of this Sundering was the creation of the Tablets of Fate, wherein AO inscribed divine reality as it existed in both worlds: in Toril, the tablets list the names and purposes of the Gods in Toril as well as the Primordials in Abeir.
For all intents and purposes, the worlds were separate, and allowed to evolve on their own. Under the aegis of the Gods, Toril saw the fall of the batrachi and the sarrukh, the rise and fall of the dragons, elves, Netherese, and finally the spread of human and demihuman kingdoms. Abeir saw a far more chaotic history involving unpredictable elemental magic, rule by the powerful and destructive Primordials, and the emergence of potent races of beings such as the genasi and dragonborn. These creatures existed in limited quantities in Toril (the consequence of planar travel or the occasional cross of genies and dragons respectively with humans), but in Abeir they flourished and built kingdoms all their own.
Then came to pass an event in Toril known alternately as the Avatar Crisis, Godswar, or the Time of Troubles. Bane, Bhaal, and Myrkul (three of the younger gods who had not yet ascended when Ao sundered the worlds) stole the Tablets of Fate, thinking them key to great power, perhaps even control over the Overgod himself. Their schemes were eventually thwarted, all three slain, and the Tablets returned to Ao. The Overgod decreed that the Tablets clearly meant nothing to the gods, and so he destroyed them and left the deities to their own devices in the chaos that would soon ensue. This began the unraveling of Ao’s great Sundering through the time called the Era of Upheaval.
This era (lasting from 1357 through 1486) was marked by extreme turbulence, from the invasion of the Great Kahan to the fall of Cormyr’s King Azoun IV, from the rise of Cyric and the rebirth of Bane to the silence and empowerment of Lolth, to the Rage of Dragons and the Reclamation of Myth Drannor, and finally to the death and merging of gods and the unraveling of the Weave of Magic. This last event touched off a great mystical curse upon the world called the Spellplague, which would reshape the world. The Sundering fell apart with the Weave, and pieces of Abeir merged with pieces of Toril and vice versa. The world was truly in peril and in need of great heroes to save it.
Then, as the 15th century came to a close, the third and final Sundering envisioned by the elven prophet so many years ago would come to pass. Ao would once again forge the Tablets of Fate, inscribing the names and purposes of the gods he chose to serve in a new, inclusive divine reality, free of the petty schemes of unchecked gods. The worlds Abeir and Toril would be split from one another once more, though both would carry echoes and marks of the experience. Many of the gods lost to the ravages of time would return, reawakened to fulfill their inscribed purpose. Ao would end the Era of Upheaval and reforge Toril as it had existed before the series of cataclysms brought on by the actions of the gods. A new world, true to the old and moving ever forward, would dawn, and heroes would once more be called to prevent such a cataclysm from occurring ever again.