Netherese
The Netherese were a race of humans living in the area that is now the Anauroch (northwestern faerun), famous for their works of stupendous magic. Netherese practitioners of the arcane arts, known as Arcanists, reached staggering heights of power unknown to wizards today. The mightiest of them wielded spells that could raze miles of countryside, or extend lifespan indefinitely, or even shear off the tops of mountains and levitate them permanently in the air to serve as the foundations for flying cities.
In this latter endeavor, they were greatly aided by the invention of the Mythallar, a spell engine that can continuously and indefinitely produce magical effects on a large scale (such as, for example: levitate on a mountain). This wonder was the brainchild of one Ioulaum, arguably the greatest Artificer who ever lived. Many would copy his works, making flying cities of their own, and battling rival Artificers for dominance and prestige.
In the end, the Netherese empire could not bear the weight of the Arcanists' hubris. One among them, a prodigy who might have eclipsed the great Ioulaum, was named Karsus. He wove a spell unlike any other, a spell that could slay a god and allow the caster to absorb the god's power and divinity. Perhaps the spell might have worked on a lesser deity, but he chose to target Mystryl, goddess of magic, one of the mightiest of gods. When his spell landed, she was indeed slain, and with her death came the disruption of all magic in the world--including the magic holding aloft the great flying cities of Netheril.
Karsus, now blessed with Mystryl's power, was unequal to the task of maintaining her duties. He could not stabilize the flow of magic--nay, he was instantly driven mad and his spirit torn asunder by the raw power he had stolen--and without a master, the magicks of the world went wild. The cities plummeted to the earth, and the Netherese civilization was destroyed overnight.
Mystryl, however, had foreseen her own demise, and had cleverly found a vessel for her power. A human woman, a powerful priestess of the former goddess, had been selected before her very birth and groomed for the task. She stoody ready when Karsus fell, and with the aid of a great ritual, she took command of the Weave and inherited the mantle of Mystryl's power. Magic worldwide was stabilized--too late for the falling cities. And having seen the destruction men could wield with magic, she would go on to enforce limitations on the use of magic henceforth, making it nigh impossible to weave spells of the magnitude used by the Arcanists.
That is, until she died in 1358.