The lore of the Forgotten Realms has often been written as though the Realms were all that was; it does not appear that many Realms authors defaulted to seeing Toril as just one world among many. Thus, there are many lore inconsistencies, particularly when it comes to Mystra’s control of magic, when we compare Forgotten Realms lore and wider multiverse lore. To wit, within the Forgotten Realms, Mystra is among the most powerful of gods, and controls all magic for all creatures great and small.¹ Outside of the Forgotten Realms, Mystra is a minor player from a single sphere who doesn’t control much of anything outside it.
So you ultimately have to take a stance on this meta situation before you can start to untangle anything about this question. Are we coming from a Realms-specific mindset, where Mystra is a chief god? Or are we coming from the multiverse, from a Planescape mindset, where Toril is just one world of no special note, and being a god found only there, Mystra is thus a relatively minor player?
On the subject of the end of “true” 10th-level and higher spells, for instance, we have two conflicting accounts. The multiverse-centric version has all of the changes from 2e to 3e caused by the events of Die, Vecna, Die!. Here, the lich Vecna gets himself misted by Ravenloft, and becomes a Darklord of Cavitius, one of the domains in the Demiplane of Dread. However, he schemes to absorb the divine power of rival and demigod Iuz, which causes Ravenloft to reflexively boot him out, and he manages to set things just right so that he is booted out of Ravenloft into Sigil while still mid-apotheosis. He uses this loophole to get around the Lady of Pain’s ban on gods in Sigil, and then attempts to take Sigil for himself as part of a wider scheme to take over the multiverse. Canonically, adventurers succeed in ousting him, but the damage is done, and all the changes from 2e to 3e—including the loss of true 10th-level spells—results:
Some Outer Planes drift off and are forever lost, others collide and merge, while at least one Inner Plane runs "aground" on a distant world of the Prime. Moreover, the very nature of the Prime Material Plane itself is altered. ... New realms, both near and far, are revealed, and realms never previously imagined make themselves known. Entities long thought lost emerge once more, while other creatures, both great and small, are inexplicably eradicated. Some common spells begin to work differently. The changes do not occur immediately, but instead are revealed during the subsequent months. However, one thing remains clear: Nothing will ever be the same again.
(Die, Vecna, Die! pg. 151)
The problem with this story, from a Realms perspective, is that none of this has anything to do with the Realms. Vecna and Iuz were Greyhawk characters, and Sigil and the Lady of Pain are of course central to Planescape. And the Demiplane of Dread was yet another campaign setting. If this were to be the canon explanation within the Realms, it meant anyone learning about the Realms would also have to learn about all of those settings. It also meant that the grand battle between Mystra and Shar starts looking like a minor scuffle between two locals with little influence on the broader scheme of things, which is probably not really the tone Realms authors were looking for. So as a result, the 3e Forgotten Realms Campaign Setting defines an entirely new cosmology, the World Tree instead of the Great Wheel. Almost everything about the World Tree is identical to the Great Wheel, with the key difference being, quite simply, that it wasn’t the Great Wheel, and events on the Great Wheel had no influence on the World Tree (or vice versa), and thus had no influence on the Forgotten Realms. The other difference was that, although all the other planes were the nearly identical to those in the Great Wheel (per Player’s Guide to Faerûn), they didn’t connect to one another directly—you had to go through Toril, the “trunk” of the tree, in order to get from one to another. This centralized the Realms, justified the importance of its gods and other denizens, and, conveniently, replaced the central pillar of the Great Wheel: the Spire with Sigil atop it.
That meant that Die, Vecna, Die! was—kinda sorta—no longer the explanation for the edition change in the Forgotten Realms. As ZwiQ’s answer about edition transitions in the Forgotten Realms says, the late 2e FR module Cloak and Dagger establishes a timeline from 1368 DR to 1370 DR, and mentions a few things for 1371 DR onwards. The 3e Forgotten Realms Campaign Setting picks up where that left off, stating the campaign runs in 1372 DR. There is no “transition event” specified here. ZwiQ mentions Die, Vecna, Die! at this point, because at least from Planescape canon, that’s the explanation for everywhere, FR included. From an FR (only), World Tree-based, perspective, perhaps not?
So what they did instead was a ret-con. Mystra’s ban—established in the 2e module Secrets of the Magister as thoroughly detailed in illustro’s excellent answer—was changed to be more thorough than it had previously been. Where Secrets of the Magister talked about Mystra’s ban just changing the incantations and symbols for some 10th-level and higher spells so that old codexes were no longer accurate, thus forcing spellcasters to engage in an expensive and risky research and experimentation process, Lost Empires of Faerûn turns Mystra’s ban into something much more comprehensive:
Mystra banned certain high-level spells that she deemed too powerful for mortals to wield responsibly. Thus, current-day spellcasters no longer have access to true spells of 10th level and higher. Instead, access to epic magic comes via two feats—Improved Spellcasting Capacity and Epic Spellcasting—that function in very different ways.
The Improved Spellcasting Capacity feat grants spell slots above 9th, theoretically without limit. Before Mystra’s ban, powerful mages could fill these spell slots either with true spells of 10th level or higher, or with 9th-level or lower spells fortified with various metamagic feats. After Mystra’s ban, only the latter option remained available.
The Epic Spellcasting feat allows the development and casting of epic spells (spells requiring the use of the epic spell system presented in the Epic Level Handbook, cast through superior mastery of the Spellcraft skill). Mystra’s ban has never applied to epic spells, even those that duplicate the effects of 10th-level and higher spells developed before the fall of Netheril.
Epic spells did exist during the days of Netheril, but the Netherese largely abandoned their development after the discovery of the nether scrolls. During the age of Netheril, epic spells were largely the province of the Fair Folk and were thus almost exclusively associated with elven high magic. The Netherese, for their part, preferred to develop and use 10th-level and higher spells.
(Lost Empires of Faerûn pg. 43)
Here we see how 3e is being handled as ret-con rather than in-world change: elven high magic always existed, but the Spellcraft skill and epic spells of the style that the Epic Level Handbook describes were new. Likewise, the mention of metamagic feats is a ret-con; those weren’t an option before 3e. Lost Empires of Faerûn describes the Realms as if things were always this way.
Anyway, later editions quietly dropped the separation of the Realms from the rest of the multiverse. Hell, for that matter, 3e and “v.3.5 revised edition” Forgotten Realms material often seemed to forget about it too. Officially, the World Tree is regarded as an alternate model of the multiverse, something mortals contrived to try to understand the planes, but which has not stood the test of time and which has largely been outcompeted by the Great Wheel model. This contrasts with 4e’s World Axis model, which—at least in FR—was treated as an actual change to the layout of the multiverse that occurred due to the Spellplague, and which was undone by the Second Sundering. Other campaign settings didn’t bother to explain why the planes were different, and post-4e have largely elected to pretend the World Axis never happened.
On the other hand, later editions have also exported a number of formerly-unique-to-the-Realms details to the wider multiverse—most importantly, the Weave, which previously did not exist outside of Realmspace. How these things work now that they’re no longer exclusive to the Realms—and how much influence Realms deities have over these things outside of the Realms—is not explained at all, however.
Is it possible to cast spells above 9th level in a crystal sphere that is not under the influence of Mystra?
Originally—in 2e—yes, this was true. But then Mystra’s ban wasn’t absolute in 2e FR, either.
After the events of Die, Vecna, Die! changed the way things worked from 2e to 3e—including the removal of “true” 10th-level or higher spells—it became impossible to use true 10th-level or higher spells everywhere in the multiverse. However, within FR, the explanation for this may not be Die, Vecna, Die!, but rather a ret-con of Mystra’s ban making it more absolute.
- Ignoring, for the moment, Shar’s Shadow Weave.