Karsus was the Netherese magus who fucked up everything real bad and ruined magic for everyone forever. Before his eponymous Folly
there was much less limits on magic, and the tyrannical and continent-spanning Netherese Empire had grown strong and prosperous through mastering it -- controlling the world through giant floating cities for centuries. Then Karsus, an arch-mage among arch-mages, performed a ritual he had designed to cast a spell that would allow him to supplant Mystril (the original goddess of magic) and become a god in his own right. And, well, he succeeded, in part, but not in the way he had expected. He stole most of her power, but was unable to control it, and magic went haywire everywhere. Sacrificing her own life, Mystril cut the ability to access the weave from the world in order to keep Karsus from destroying the world through his failure, and functional magic did a big stop.
The Netherese Empire, being dependant on said functional magic, fell in an instant -- their flying cities literally falling to the ground, killing tens or hundred of thousands, and causing untold natural and magical catastrophes as they crashed. Karsus was destroyed, or turned to stone, or something, I don't remember.
Mystril was reincarnated as Mystra, the new goddess of magic, and magic was restored to the world. She decided that she was never again going to let people have access to that kind of magical power, however, and rewove the rules of magic so that magic just couldn't do that kind of stuff any more.
Even though I mostly said Gale was Karsus as a joke (though I definitely believe it is more likely than him being Elminster) I think that whatever he did to cause his conditions was something he shouldn't really have been able to do by abusing forbidden Nether knowledge of some kind that he might have gleamed from off of Mystra. Likely something similar to Karsus' Folly i nature.