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The Forgotten Realms was first published for AD&D 1st edition. When 2nd edition came out, the change in spells and available classes was explained through the Godswar aka The Time of Troubles.
The introduction of D&D 3e was exceptional in that there was no overarching, global in-setting event introduced to explain the rules changes. Oddly, the adventure Die Vecna Die! was intended to explain the rules changes (for all D&D settings), but its events were never made FR canon. WotC just proceeded with the 3e rules and setting changes without adding any corresponding historical events. The 3e change is thus “silent” in FR's historical record.
Karsus's Folly was made global setting canon with 3e as a historical detail that further developed the history of Netheril and explained some changes local to the Anauroch and some new game elements, but it wasn't the same kind of global upheaval—it was more of a retcon slipped in as undisruptively as possible.1
The Spellplague (and about a century) explained the shift from D&D 3.x to 4e, and the Second Sundering did the same for the transition to 5e.
- Although Karsus's Folly was first described in a 2e adventure, the way 2e setting publishing worked meant that only the frozen-moment-in-time documented by the basic setting boxed set was canon. DMs were given explicit authority over what was canon beyond that. Thus changes made by adventures were only canon in a campaign if a DM used those in their campaign — because of course, the PCs might change the outcome! D&D 3e cemented Karsus's Folly as the official way it happened in Realms history for that revision of the base setting, so though it took place earlier, it wasn't canonised until 3e.
Ao created tablets of new rules intended to restore peace and balance to Toril.
In a video of a 2012 Gencon panel featuring Forgotten Realms novel authors, What is the Sundering (Part 1), it is stated that the supreme overgod Ao intentionally re-created the tablet of rules that separated the worlds of Abeir and Toril, intended to restore balance and order to Toril.
The events of the D&D 4th edition era where the now twin worlds of Abeir and Toril were recombined into one world are revealed to be the result of Ao destroying tablets of rules he created, those that divided the worlds in the first place long ago. Originally, their purpose was to divide the world into Abeir, where the Primoridials held sway, and Toril, where the Gods had power.
However, when two deities stole the tablets and began fighting amongst each other, Ao destroyed them to teach the gods a lesson, and the worlds were recombined. Unfortunately, this only made things worse, and so Ao set about recreating the tablets and dividing Abeir and Toril in two once more, but this time with a new rule limiting the ability of the gods to damage Toril in their petty squabbles.
Ao's reasons are described at 11:35 in the video:
Once it is over, the word of Ao declares: The Era of Upheaval is ended. Great stories remain to be told in this era, but they are not the stories of gods and godlike beings. They are the tales of mortal heroes, taking a stand to preserve the world they love.
This ethos reflects Wizards of the Coast's policy for the focus of Realms products and novels, which we learn at 23:20 in the video is intended to return to "the heart of high fantasy", and at 31:15 it is stated that WotC wants individual characters to be significant actors (e.g. why player characters should have to save the day instead of leaving it to deities).
As such the novels of that era will focus on the actions of people, not the gods or Ao. It may be that no novel or sourcebook explicitly references this lore about Ao stated by Wizards of the Coast. At 36:48 in the video, it is said to be likely that no protagonist of these novels is likely to be fully aware of the situation with the gods, and even Elminster may only partially understand it.
It should be noted that gods can, to some extent, defy Ao. It was gods who stole the tablets from Ao in the first place, and Tiamat has a loophole which allows her to be summoned in person to Toril. This suggests that Ao's new rules are significant, but not necessarily absolute.
Best Answer
Well then, let's go through the editions. In all versions of D&D, the holy sword is a magic weapon (with high "plus" value) that grants extra powers to a Paladin:
Before Faerun existed, a Paladin's holy weapon was nigh-invulnerable against spellcasters. In the early Realms it became weaker, but arguably still the greatest non-artifact weapon. After the Time of Troubles, magical properties (including Holiness) became modular and commoditized. Then the Spellplague turned everything fiddly but precise, balanced and rectangular (perhaps the Modrons were behind it). Since the Second Sundering, most items show homage to their pre-Spellplague instantiations. Interestingly, the Holy Avenger even resembles its original form from the old world, albeit much more bounded.