The Spellplague quickly spread from Toril to many other crystal spheres. Each of the Crystal Spheres that caught the plague would have a azure fire burn on the outside of the Crystal Sphere for between six and twelve months before the interior began to be affected. It seemed to spread along the rivers in the phlogiston, and it initially only affected small spheres with little magic. The Spellplague did not affect every sphere like it did Toril with massive upheavals of the earth and countless dead. Others just had their magic fundamentally change and life went on normally.
It was like that on Eberron, the first of the heavily populated spheres to succomb. The Spellplague swept over it and few died. Though the fundamental changes were apparent, the world seemed to go on as if nothing had happened.
The next crystal sphere to become infected was Athas. Life there has always been hard and the mass die-off of mages that resulted from the Spellplague was seen as a triumph for the blasted land.
Krynn was the first major Crystal Sphere to catch the Spellplague that was connected by trade to Toril. The damage and changes wrought by the plague were devastating, but nearly so as on Toril. Thousands died because of it, but the Spellplague wasn’t the world shaking cataclysm that it was on Toril.
Oerth should have been one of the first crystal spheres to be infected. It was a major caravan stop on the river between Toril and Krynn and the azure burned on the outside for four months before it miraculously disappeared. Something happened that stopped the horrible plague from infecting Oerth. Whatever was done must have been a monumental task for a group of selfless adventurers. A task on this scale would have had no reward other than the thought that the world was saved. Truly, it was a quest fit for great heroes.
It was in fact done by Iggwilv, evil wizard, adopted daughter of Baba Yaga, beau of Graz’zt, mother of Iuz, and now savior of the world. Her notes on the ritual that she performed to inoculate Oerth were included in various sections of the Demonomicon.
Once all the pieces of the Demonomicon are collected by the party, the same ritual can be done on Aerth, stopping the Spellplague from ravaging the land.