As Above - So Below
It was created in -2954 DR by the archwizard Ioulaum, sixty years after he had invented the mythallar. The creation of Xinlenal led many other powerful mages of Netheril to create flying enclaves of their own, in effect splitting the nation into two, High and Low Netheril. In -339 DR, the Year of Sundered Webs, Ioulaum suddenly and mysteriously abandoned his enclave. The sudden disappearance of their great champion caused wide-spread panic in Netheril, leading to the casting of Karsus' Avatar and the end of the Empire of Magic.