Dear Vic
Let me be perfectly honest with you... I do not know what the Spellplague is. It is NOT explained in The Orc King and as far as my internet search goes, no one knows exactly what it is.
That doesn't mean I don't have a guess.
I think the Spellplague is basically our world's "Black Plague" that only affects those attuned to the Weave.
Let me show you this interesting forum post I found. The writer has no inside information (as far as I know) but is very perceptive to the changing realms.
"Now we have the Spellplague, from what we know it's only () going to kill spellcasters in the Realms, many of whom are well known and even well liked NPCs and flavor characters. All in order to justify a radical magic system change ('meh') and to address a perceived dislike of the Realms' many wizards ('bah'). That last one really has me worried. Is this really necessary? Trying to fix what some call a problem in an almost punishment, wish-fulfillment sort of way. Let's just magically wipe the characters some people say they don't like out in a horrible plague. It's the sort of thing you wouldn't even wish upon your enemies. Am I off the mark? I really hope I'm wrong about this."
"Yes, I'm being a bit snarky, but I hope my statements tapped into some logic and wisdom. All I can hope is that the Spellplague will not utter obliterate the setting and hope to reconstruct it again. It is ridiculous easy for WotC to do the former but practically impossible for them to accomplish the latter no matter how hard they try. I feel the Realms' popularity is a bit of a gestalt phenomenon. A number of varied factors came together at the right time at the right place to give WotC this popular setting. Trying to fix things by taking the setting apart, dissecting it, and them putting it back together again will likely not yield the same success. This thing has a life of it's own, it will do well naturally, but if you rip it up and slap it back together you get something closer to Frankenstein's monster, a simulacrum of the real thing, resembling it but only a diminished version of the original."
WoTC is in production of their 4th Edition D&D. Many many people are worried that, in order to justify a "radical magic system change" they are going to kill off all the powerful heroes (probibally in order to "re-start" the realms as it were). As most people would say, it is a very stupid and foolish attempt, if that is what it is. Just like the Time of Troubles, only this one much worse. Elminster, Allustrial, Manshoon, the Red Wizards of Thay... it is possible that WotC seeks to wipe them all out in one fell swoop. To clean the slate for their new 4E marketing campaign.
I honestly, to any and all Gods that are listening, hope I am completely wrong.