"The Before times, when the gods were torn from us."
This sort of information is generally known to all educated people, although many may dispute some details, the general history of the world is well-known-- after all, some elf was probably there, and then spent 12 years writing a poem about it.
Not much is known of the pre-Sundering times, as scant written or historical records exist. If you need to know something from this time, the best creatures to ask would either be gods, spirits, or dragons, and none of them will likely want to talk about it.
The age when the former slaves of giants and dragons, children torn from their gods, could finally begin to rule themselves.
“I am going to go into this world, and I don't care if you and your narrow mind cannot be a part of it.” ― Elise Kovan, wild sorceror
"...and they came from the wild places, seeking vengeance for being driven to such inhospitable land, burning and reaping as they came..."
"The king went down to the wishing well, the wishing well
He wished for a balm to end his hurts
The king came back from the wishing well, the wishing well
And all he could feel was thirst."
- Arcadian children's rhyme
The kingdom of Dusk fell to a sudden unexpected undead threat-- an army of undead warriors, displayign rare intelligence and led by a terrifying figure known as the Cursed Lord-- systematically destroyed the kingdom of Dusk. The Sun-king Erasmus fell in defense of his capital, Sunset. Most of the surviving population fled into surrounding nations as refugees. The Cursed Lord was defeated, unbeknownst to many, by a party of adventurers who halted his advance just before he was poised to strike over the mountains into neighbouring kingdoms. In the aftermath, the kingdom of Dusk remains occupied by a now leaderless army of undead.
A party of high-level adventurers is contacted by the Riftwalkers, notably Lander and Lady Whisper, and asked to fly to Sunset, former capital of Dusk, and shut down three portals that have opened there, allowing fiends to pour forth into the material realm. They travel to Sunset, defeat many powerful foes and close three planar gateways, including one anchored in the Abyss itself.
At the third gateway, the party refuse a deal with the archdevil Hutijin, but in the process allow him access to a powerful soulstone. Only the intervention of a powerful celestial, Calisto, prevents catastrophe.