Skies Over Cormyr
The short version is that Netheril is the name of an ancient empire that once occupied, and dominated, most of the known world. Centered in the Anauroch, the Empire of Netheril was one of the earliest human civilizations. Magic was commonplace, and almost every citizen had SOME amount of magical ability. At one point, as many as 9 flying cities, each built on the inverted flying upper half of a large mountain, crossed Faerun.
Pride goeth, however, and there are some that don’t care for humans. One race, called the phaerim, began waging magical war against Netheril…and winning…damaging the Weave in the process. One archmage took it upon himself to attempt to repair the Weave, and in the course of casting his spell caused the entirety of magic across Toril to falter in the process.
The archmage died, and 8 of the 9 floating cities came crashing to the ground. This was the end of the Empire of Netheril as we know it. Survivors escaped to the south, some eventually settling into Halruaa.
Until, approx 450 years ago, the 9th city popped back out of the Plane of Shadow where it had been stuck, still flying, astonished at what had happened in the last lots of years. These Shadovar, as they came to be called, watched the world for nearly 20 years before acting to restore the Empire of Netheril.
The Netherese crushed the Zhentarim strongholds in the Moon Sea, rebuilding these as outposts, seized Sembia as a puppet state, and attempted to conquer Cormyr. They also began terraforming the Anauroch desert, trying to turn it once more into the green verdant farmland that had supported them so many years ago.
After 150 years of attempting to take over most of north central Faerun, and succeeding, the Empire of Netheril disappeared again. This time, however, the city state didn’t go with them, rather the entirety of their shade-touched society just up and vanished. While collaborators were left behind, any who had been granted shade-touched longevity vanished.