Well here's the low down on the "spellplague" before it even was the "spellplague" from Ed himself. Given that the 100 year jump and the Abeir returning thingies are now in the public domain, this post isn't spoiling anything and I thought that the wider forum community might appreciate the insight.
There are actually THREE references, all of which will really only be understandable "after the fact." One of them is the "Smashed the black star" curiosity that several scribes have been so fascinated by, and another is a CYCLOPEDIA heading (yes, just the heading). [Ed was referring to the "Abeir-Toril" heading]
The third, "main" reference is on page 40 of the DM'S SOURCEBOOK, referring to Mellomir's claim of finding an "ancient" and "magical" treasure in the vicinity of the Haunted Halls. The dwarves vanishing over the years were fleeing the Realms through gates (portals, linking to other worlds / alternate Prime Material Planes) that are not in the Haunted Halls, but can be reached through the Halls (I think the maps I gave you at GenCon included the "Whisper's Crypt" level, yes?). The dwarves were departing to avoid the coming Spellplague, and the monsters and adventurers vanishing into and emerging from the Halls were also using the gates.
The original text of this entry had Mellomir referring to an unspecified "message of fire."
The original turnover of FR1 THE HAUNTED HALLS OF EVENINGSTAR included a magical "message" formed by flames dancing in midair that briefly appeared to anyone entering the many-pillared room of Encounter 31, that said this: "A great storm of magic is coming, that will twist or maim many. Beware the Black Star." It was edited out because the TSR designers of the day thought it too substantial to print without providing something of an explanation (that there was no wordcount left in the module to provide, and that would open a can of worms they wanted to avoid). This same message appears in front of the gates used the dwarves, monsters, and adventurers, whenever they are approached from the "Realms" side.
So there you have it. Too mangled and chopped-down for anyone studying the Old Grey Box to recognize without this explanation. Sorry. I DID warn everyone it would be nigh-impossible to spot, beforehand.
By the way, there's something else in FR0 tied to this: Sabirine's Specular. A Spell Engine provides a tiny "shelter" from Spellplague effects around itself, and Sabirine chose to die rather than embrace lichdom because she didn't want to endure years of undeath only to be swept away in the Spellplague, or twisted and maimed and left in a world ravaged by the Spellplague.
The Spellplague term isn't mine (I called it "the Doom," or "the Coming of the Black Star" or "THE Spellstorm"), and I always intended this to be a mythical always-in-the-future bogey-tale.
There you have it.
-- George Krashos