After facing defeat and death by Tiamat at the Well of Dragons, the group was raised by True Resurrection spells by Laeral and a few clerics. She told them about the hell on earth that was created by Tiamat up and down the Sword Coast. But, some scholars, including Maccath from the Arcane Brotherhood and the wizardess Sanura Hadad from Neverwinter had gone through the books, documents and scrolls that were found both at Arauthator’s lair on the Sea of Moving Ice and at Xonthal’s Tower. Tomes that spoke of a great wizard that also studied time; Acererak. Maccath deduced that Acererak had an artifact named the Orb of Time that the PCs could use to return to the Summoning Chamber and try again to halt the ritual, therefore stopping Tiamat from entering the realms.
48 hours after the defeat at the Well of Dragons, Acererak turned on the Soulmonger creating the Death Curse. Was it planned? Probably not but we really do not know. Could Acererak see if Tiamat would be summoned or not? We do know he is excellent at setting up contingencies and more than likely he saw an opportunity and went for it. So, those that were able set off for Chult to try and find the cause of the Death Curse and eliminate it. Many adventuring groups tried and many died. One group was finally able to figure things out. They entered the Tomb of Annihilation and found an alien creature hooked up to a machine that took souls and pumped them into the creature in order to create a new god, one to be controlled by Acererak. The group destroyed this machine, called a Soulmonger, freeing the atropal. Soon after they were confronted by the lich himself and they were almost wiped out. At the last minute a group of Red Wizards of Thay arrived assisting in the defeat of Acererak but not his destruction. He teleported away when the numbers were no longer in his favor.
We tracked him after his defeat on Chult to his last known haunt: a tomb in the Serpent Hills, several hundred miles southeast from Waterdeep. Go there, find the Orb of Time, travel back in time, and stop the ritual(s).
As for the Faerun specifically the Sword Coast, things are not well.
It is estimated that 75% of Waterdeep was destroyed. It seemed Tiamat’s full wrath was sent to Waterdeep as it was the headquarters for the alliance against her. Other cities did not fare as well. Neverwinter was not destroyed but 50% of its population is dead or missing. Candlekeep is 95% destroyed, the library burned to the ground. Thousands of years of knowledge lost. No contact with Daggerford. It is thought that it is no longer on this plane.
Phandalin also felt her wrath brought down on it. Not sure who survived but the town is destroyed as well as the Manor House. No one has been able to get into the mines so not sure what happened to the Rockseeker brothers.
The Tomb of Horrors awaits.