This thread has inspired what will likely be my next big campaign.
The Fallen Realms
They were called the Heroes of Greenest, and when they left Waterdeep 20 years ago to lead the war against the Cult of the Dragon and to stop Tiamat from entering the Realms, the people cheered. But they had been deceived.
Led by the Edritch Knight Vorak Migad, the heroes did indeed make it into the heart of the mountain, but instead of confronting Tiamat, they swore fealty to her and helped to destroy the combined forces of the realms that had gathered in this fight against evil. Some say Vorak had been corrupted by the dark sword Hazirawn, others say his soul was always black as night.
They moved quickly on Waterdeep, Tiamat creating a lair for herself under the city, while Vorak and his three lieutenants first took over the city, and then moved out to kill any other "Hero" who might exist to challenge them. Evil dragons, empowered by their god hunted their metallic kin to near extinction.
The cult consolidated it's power over humans with a simple choice, join us and live relative comfort, work for us in squalor, or die. Halflings were pressed into slavery on farms, while Elves were simply hunted and killed on sight. Whole villages and cities, including the famous Baulder's Gate, were razed by dragon breath if they showed even the slightest resistance.
The Giants, meanwhile, began their own uprising, and soon joined with the Orcs, enslaving the dwarves.
An elemental cult, gained power quickly and eventually found alliance with Thay, who had begun their own expansion into the Sword Coast, though when the being they summoned went out of control and began laying waste to the countryside, it was the Red Wizards themselves that put down the cult, and eventually retreated back to Thay.
And then the Demons erupted in the Underdark. With no heroes left to fight such an evil, they quickly took over the under realm and make their way to the surface when they grow hungry. Rarely do the Demons hold territory they win on the surface, instead happy to feed for a tenday or two on the villages before Tiamat's forces come to force them out.
It is a dark time for the realm, and yet there are whispers of resistance and rebellion. Some say that the Harpers and Emerald Enclave still exist, and still recruit those who show bravery against the cult to the cause. But if such rumors are true, it is a small hope for those who now live under Vorak's oppresive yoke and the fear of the demons below.