These two major catastrophes define both of these fantasy realms. In both, the greed of a powerful mage brought doom upon mankind by annulling a god's powers, bringing down cities (literally!).
sources:
http://forgottenrealms.wikia.com/wiki/Karsus's_Folly
http://dragonage.wikia.com/wiki/First_Blight
For example, Neverwinter Nights: Shadows of Undrentide takes place in the ruins of a fallen city resulting from Karsus' Folly, similar to some explorations you have in Dragon Age of Blight stricken ruins of old.
Neverwinter Nights 2 almost solely focuses on a related event: the major plot point is an avatar that turned into the Shadow Weave (and thus was corrupted) at the moment of Karsus' Folly. Not unlike the fates of the 'Archdemons' and their corruptions in Dragon Age lore (affecting entire realms and hordes with them).
'The Fall' is of course a common trope. We are likely to see it again, for example, in Torment: Tides of Numenera. [There. I'm making a prophetic guess. You first heard it from me.]
Let us admit: one reason the BG saga is so popular and powerful is that it is held together by a similar (though lesser) catastrophe (The Avatar Crisis, where Bhaal and others walked the land). This is also why IWD/IWD2 are not as compelling (fine games though they are) because the stakes are not that high. (A powerful demon terrorizing a snowy landscape is small stuff compared to the Weave seizing to exist through the folly of a mage challenging the God of Magic!)
(I really would have wished to see a story describing these catastrophes themselves, rather than merely the aftereffects.)
Comments welcome.