Ok when AD&D switched from 1st to 2nd edition, they had the Time of troubles which reflected into gameplay by loss of a class, some gods, and spells changing their effects. 2nd to 3rd...I have no idea what happened there. 3rd to 4th, they had the spellplague. Could someone tell me what happened between 2nd and 3rd? Because they always reflect major changes in the game with some sort of catastrophic event in the story.
I'll go with "Bullshititus". They have yet to come up with anything even remotely resembling a good reason for the radical changes to a game system.
Now, you look at every other company, they don't even bother trying. They just put out the new system and say "History's all the same and shit, but these rules kick way more ass!"
A wizard did it.
The war of the bhaalspawn? :D
anyway..
I'm not entirely certain they bothered to write an event that changed everything. but if there was ever a place to find out, here it is: http://forgottenrealms.wikia.com/wiki/Portal:History (http://forgottenrealms.wikia.com/wiki/Portal:History)
Apparently there was no major event between 2nd and 3rd. The bhaalspawn thing was really kinda downplayed as it was not something that effected a large population of the world and it was really not common knowledge. Honestly, It felt a bit like a highlander thing. All the bhaalspawn wiping each other out to get to being just one or something like that, and no one really knew about it.