Centuries ago the Netherese Empire ruled over all of Faerun. What made the Empire so indomitable were two major factors. The first was that all Netherese arcanists possessed an aptitude for magic that far surpassed the greatest archmages at the time. The second was the discovery of the Nether Scrolls. Despite all this, their empire still fell into ruin.
The city of Ythryn currently lies beneath the Reghed Glacier. It is unknown why the city fell there.
The Nether Scrolls are an ancient set of five artifacts once possessed by the Netherese Empire. It is not currently known what knowledge is hidden in the scrolls. The cases of the scrolls are made of pure gold yet they remain practically weightless. There is writing on the case, however using magic to try and decipher it causes intense headaches. This is because of a safeguard that Mystra put in place to prevent anyone from deciphering the Netherese language. Her exact words were "From now until the end of existence, so say I, no soul living or unliving shall posses the ability to learn such a heretical language." She made this decree shortly after the empire was annihilated. After the fall the scrolls disappeared, most of their whereabouts are currently unknown.
The first Nether Scroll found was beneath a church of Amaunator, the Netherese god of the sun. The writing on the outside of the scroll case is untranslated
The second Nether Scroll is believed to be in the city of Ythryn, deep beneath the Reghed Glacier
The locations of the third, fourth, and fifth are unknown
It isn't known what happened to cause the fall of the Netherese as now nothing remains of their once great empire. It is said that the sages of Candlekeep hold knowledge about the Netherese that is inaccessible to the public.