Karsus's Folly

Karsus's Folly was a climactic event that caused the end of the Netherese empire in.

History
After decades of war with the Phaerimm, the homeland of Netheril was quickly becoming a wasteland, partly due to the strange help the Netheril got from the Sharn. The spells of the sharn altered the very geography of the land when it came in contact with the phaerimm’s magic drain and life drain spells, transforming mountains into rolling hills or increasing the spread of the desert.

When Ioulaum suddenly disappeared, panic surged through Netheril. Karsus, feeling the responsibility for Netheril's future on his shoulders, felt that it was his duty to weld the nation back together. Therefore, he tried to wrest the mantle of divinity from Mystryl using the spell Karsus's avatar. Immediately after he cast the spell, his body swelled with the sudden influx of godly power, and his mind filled with unimaginable knowledge. He instantly realized the horrible mistake he had made.

Mystryl was the only being with the experience to fix the damage that the war between the Netherese and the Phaerimm was causing to the Weave. When she lost her ability to keep the Weave intact, the inundation of magic surged and fluctuated, and the effects of all things magical doubled for a short time. To save the weave, Mystryl sacrificed herself and, in the process, broke contact with Karsus. This caused all magic to briefly cease functioning.

Without the magic, the floating cities of Netheril could not hold their positions in the skies, and most of them crashed to the ground. The archmage Karsus died when the link to Mystryl was cut and his body was petrified. In the last moments of life, his stony eyes, still shimmering with the last glint of godly omniscience, caught a glimpse of the cities of Netheril smashing to the ground, killing all. His heart broke—greed for the power of the deities themselves had caused the destruction of his home, his family, his friends, and his people.

After Mystryl was slain, the goddess of Magic reincarnated within moments as Mystra, and her first priority was to recreate the Weave of magic. She established new rules for using magic, and no spell of 10 level or above would function. When she had done this, she was able to save only three of Netheril's floating cities, while the rest were utterly destroyed. The cities Anauria, Asram, and Hlondath were high enough above the ground to survive the fall long enough that Mystra could land them safely, and the survivors stumbled away from the cities in shock. Priests and priestesses of the new goddess of magic were told the story of Karsus in dreams and visions when they prayed for spells. It was Mystra's attempt to make sure that nothing like this ever happened again. The fourth city that survived the fall of Netheril was Thultanthar, which was pulled into the Plane of Shadow to strengthen the twelve princes spell power.