The Sundering

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.
The Sundering is somehow connected to The Sundering that took place around -17600 DR when an elven High Magic ritual went both forward and backward in time to create Evermeet.
What was known is it that the Sundering signaled the coming back of many gods believed dead including Mystra, Helm, Mask, Lathander and others, and on the other side Shar losing most of her power in the wake of the defeat of the Shadovar. On a more mortal plane the city of Shade was destroyed in a fighting with Myth Drannor which also sustained heavy destruction. Telamont Tanthul and most of the Princes of Shade were killed permanently