Spellplague

The Spellplague was a disaster that befell the entire multiverse when the Goddess of Magic, Mystra, was murdered by Cyric and Shar. Mystra's death triggered a collapse of the Weave, unraveling and reshaping flesh, stone, magic, and even spacetime.  A storm of virulent blue flames swept across the face of Toril and into the planes, until it reached the parallel world of Abeir, where its devastating effects also changed that world. In an instant portions of Toril and Abeir switched places, altering the geography of both worlds, destroying entire civilizations, and displacing large populations between the worlds. Many arcane spellcasters were killed instantly when the Blue Fire consumed them. Others were driven mad by the sudden, violent alteration of the Weave. Some were afflicted with Spellscars, magical blue markings that granted them newfound powers but often at devastating cost. Perhaps the most unlucky were those who survived but were transformed into disfigured "Plaguechanged."