If killing or (in lore) replacing mystra with upset the arcane magic, and destroy the weave, then what about killing who ever owns the “shadow” weave? What IS the difference, if any, between the shadow weave and regular weave? Is it just one is evil-ish… Or is it more than that? And can you cast 12th level spells with the shadow weave, unlike the normal one? Has anyone tried?

— T.O.R_Alexander (@TORAlexander1) January 29, 2020

1)
Shar created the Shadow Weave to obey her (for the use of mortals loyal to her), but it “leans on”/is an echo of/calls on the Weave (who is Mystra herself, and a webwork spanning the world that is a way of calling on the natural forces of the world). As an echo. 2)
…of the Weave, the Shadow Weave can never be greater than the Weave, and in fact is lesser when it comes to creating and controlling magic; it's better at destroying and compelling. And as it calls on the Weave, it's subject to the limits placed on the Weave…#Realmslore

— Ed Greenwood (@TheEdVerse) January 29, 2020

3)
…by Mystra (and Ao), so no one can use it to craft or cast magics beyond what mortals can use the Weave for. Yes, powerful archwizards and archliches and even dragons have tried…and failed. Usually the spell just doesn’t work, but if the ‘wrong’ forces have.. 4)
…been unleashed, the caster is often hideously mentally and/or physically maimed, or outright destroyed. "Always wise to avoid trying to outdo Icarus," as Elminster warns we Earth types.#Realmslore

— Ed Greenwood (@TheEdVerse) January 29, 2020

1)
Shar created the Shadow Weave to obey her (for the use of mortals loyal to her), but it “leans on”/is an echo of/calls on the Weave (who is Mystra herself, and a webwork spanning the world that is a way of calling on the natural forces of the world). As an echo. 2)
…of the Weave, the Shadow Weave can never be greater than the Weave, and in fact is lesser when it comes to creating and controlling magic; it's better at destroying and compelling. And as it calls on the Weave, it's subject to the limits placed on the Weave…#Realmslore

— Ed Greenwood (@TheEdVerse) January 29, 2020

3)
…by Mystra (and Ao), so no one can use it to craft or cast magics beyond what mortals can use the Weave for. Yes, powerful archwizards and archliches and even dragons have tried…and failed. Usually the spell just doesn’t work, but if the ‘wrong’ forces have 4)
…been unleashed, the caster is often hideously mentally and/or physically maimed, or outright destroyed. "Always wise to avoid trying to outdo Icarus," as Elminster warns we Earth types.#Realmslore

— Ed Greenwood (@TheEdVerse) January 29, 2020

This is why, when the Weave collapsed during the Spellplague, the Shadow Weave ceased to be as well. Indeed!

— Ed Greenwood (@TheEdVerse) January 29, 2020