Heirs of Karsus
Karsus was the most gifted wizard Netheril had seen, but he decided to increase his power by stealing the power of Mystryl, the deity of all magic. This backfired spectacularly, forcing Mystryl to sacrifice herself in order to prevent Karsus from gaining her power (she was shortly reborn as Mystra). Since Mystryl was responsible for the regulation of all magical flow in the world, magic ceased to work for some time, and most of the floating Enclaves fell.
The fictional Heirs of Karsus believe that what Karsus did was not necessarily doomed to failure, but could have worked if he had gone about it differently. In particular, if he had stolen the powers of a different deity, magic would not have been disrupted. The Shards of Selune provide a way for him to steal the powers of Selune, who is a fairly powerful deity in her own right. Ovid claims that the delivery of the Shards to Lady Jasmine is necessary step in using Selune’s followers as pawns.