As Above - So Below
The magocracy of Netheril was one of the ancient human empires of Faerûn that lie in what is now the (magical) desert of Anauroch. Netheril was also the home of the most powerful wizard who ever walked on Faerun, Karsus, who could cast 12th level spells (although he cast such a spell only once, and it was his last spell ever). The Netherese also created powerful artifacts known as mythallars, which allow mundane objects to become magical, dramatically changing the face of the Empire (or better said the face of the upper side of the empire, as Netheril eventually became twofold: the "have", High Netherese, and the "have not", low Netherese).
People in Netheril used magic carelessly. Great wizards used spells to turn the mountains upside down, made them float in the air and built cities on them. Netheril was destroyed by Karsus's Folly: an attempt to become an avatar of Mystryl (who sacrificed herself and was replaced by Mystra), in a last-ditch effort to save Netheril from the invading Phaerimm. When this happened all magic ceased to exist for a time. Since Netherese cities floated above the surface, when magic ceased to exist there was nothing to hold the cities up. Netheril was completely obliterated in one fell swoop. This also lead to the creation of the King of Shadows, an Illefarn Guardian. The remnants of Netheril was destroyed by the phaerimm, who would have destroyed the rest of Faerûn had it not been for the intervention of the sharn. The only surviving city is Shade Enclave — a floating city which is home to the survivors of ancient Netheril who fled into the shadow plane before Karsus's folly. The citizens of Shade — the Shadovar — returned to Faerûn on 1 Hammer, the Year of Wild Magic (1372 DR), with the help of the Evereskan elf Galaeron Nihmedu.
Three Netherese successor states (Asram, Anauria, and Hlondath) survived for a while, but all were eventually destroyed by the encroaching desert. Some Netherese culture persists in the country of Halruaa, in the Shining South, which was founded by Raumark, a Netherese wizard who led a group of refugees south after foreseeing the destruction of the empire.