The Sundering, technically referred to as the Second Sundering (sometimes called the Sundering of Toril and Abeir) was a catastrophic event in the history of Toril.
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 the worlds of Abeir and Toril. In addition, Ao rewrote the Tablets of Fate. The deities were unsure of what this would do to them and their power, and made efforts with their most powerful servants to prepare for that event.
The Second Sundering is somehow connected to the First Sundering that took place around -17600 DR when an elven High Magic ritual went both forward and backward in time to create Evermeet.
The god of murder, Bhaal, is reborn in Baldur’s Gate, apparently reclaiming the murder domain from Cyric.
Unusual disastrous events hit Faerûn, including an earthquake in Iriaebor, a plague of locusts in Amn, and droughts in the southern lands, leading to receding sea levels and conflict. In the north, the Kingdom of Many-Arrows fought the dwarves while the Dalelands was invaded by Sembia, and while Cormyr raised an army to help defend the Dalelands, it was attacked by Netheril.
People begin to claim they have been “chosen” by the gods and granted special powers, some apparently for “divine purposes” and others have no idea why. These folks were called “The Chosen.”
The Chosen of Auril starts a war with Ten-Towns in Icewind Dale, and is defeated. A rebellion of the subjugated Bedine people in Anauroch occurs when they see that Netherese forces are spread thin due to the conflict with Cormyr. By this time, the dwarves of the north are defeated and the Many-Arrows armies attack Silverymoon. Towards the end of 1485 DR, the “Great Rain” begins to fall continuously around the Sea of Fallen Stars.
The “Great Rain” causes the waters to rise around the Sea of Fallen Stars, consuming much land, but stops by the end of the year, and the orcs of Many-Arrows are defeated. Myth Drannor aids the Dalelands in their conflict against Sembia. The Arcane Brotherhood and Hosttower of the Arcane return in Luskan. Cormyr finally repelled Netheril and Sembia, and Neverwinter and Waterdeep begin to clear rubble built up over centuries of neglect.
Major geological instability results in numerous earthquakes and volcanoes, and areas once consumed by large chasms are restored to their pre-Spellplague status. Ships arrive on the mainland continent from Evermeet, Lantan, and Nimbral. Netheril floats to position itself over Myth Drannor and attack, and the conflict causes it to collapse on top of Myth Drannor, resulting in the destruction of both.
The winters of 1487 and 1488 DR were notably very harsh, and seasons start and end much later. Prayers to deities are unanswered, but their “chosen” are still present. The League of Silver Marches disbands in the aftermath of the war with the orcs, and Sembia dissolves into city-states. The few remaining Netherese forces fight the Bedine over the Memory Spire, causing an emergence of the phaerimm.
Prayers to the gods began to be answered again, and their “Chosen” were no longer to be found, but the deities were “quieter” than before, causing the emergence of new priesthoods to try to explain the different behavior.
Most gods created many chosen among mortals, trying to gather as much power as possible, in order to be as high in ranking as they could before Ao could complete the new Tablets of Fate, sealing their status and portfolio. That gambit, however, didn’t go very well for a large number of them.
For some deities, previously presumed dead or missing, it resulted in a restoration to their status as gods (Mystra, Helm, Mask, Lathander, Bhaal, Eilistraee, Vhaeraun were examples, among others), while for others (like Shar) it resulted in a loss of power and influence. The Lady of Loss suffered quite heavily in the wake of the defeat of the Shadovar, and of the city of Shade’s being destroyed in a battle with the forces of Myth Drannor, which also sustained heavy destruction. Telamont Tanthul and most of the Princes of Shade were killed.