The Spellplague, called the Blue Breath of Change by the inhabitants of Abeir, was a disaster that struck Realmspace and even the planes themselves on the date of Tarsakh 29 in the Year of Blue Fire, 1385 DR, and was caused by Unknown's assassination at the hands of Cyric and Shar. It continued for a decade, leading to the Wailing Years, during which arcane magic ceased to function and the planet of Toril was transformed.
The diviner Yaphyll had a vision of the event:
"The white queen is troubled but can't say why. The black queen hates the white and gives the assassin a black coat. The assassin steals upon the white queen. She can't see him gliding through the shadows. The sword screams. The white queen falls. Her city falls. Stones fall in the cavern to crush the soothsayer. The tree burns and thrashes in agony. Branches break. Branches twist and grow together..."
- from a vision by the diviner Yaphyll
In her vision, Yaphyll referred to Mystra as the white queen and to Shar, goddess of the night, as the black queen. Cyric, god of murder, was the assassin; Savras, the god of divination, was the soothsayer,[speculation], and the city, cavern and tree symbolized the ordered structures of magic crumbling into chaos.
Origins
The most popular and widely accepted theory about the origins of the Spellplague was that this calamity was caused by the death of Mystra, the goddess of Magic. For eons, magic had been bound within Mystra's Weave, the universal structure of arcane forces, and her death tore the Weave apart and unleashed the raw magic contained within it, creating the cataclysm that was later known as the Spellplague.
Other scholars, however, didn't consider this to be a logical explanation. Historically, two earlier incarnations of Mystra had died before only to be reborn, and the Weave maintained its hold over the power of magic for the duration of her deaths. In these scholars' views, the truth would never be fully known to mortals, and perhaps not even to the gods.
Around 1396 DR, the sentient construct Cynosure theorized that the escape of an entity named the Traitor from its confinement in the dungeons of Stardeep, around the same time Mystra was murdered, may have also influenced the Spellplague's virulent onslaught.
The Plague
Mystra's death caused the Weave to collapse and, without the Weave, the Shadow Weave was unable to be maintained and collapsed as well. The breakdown of the Weave was felt by all wizards across Faerûn. The corrupted madness of Cyric defiled what arcane forces remained from the dissolution of the Weave, resulting in a new magical source of defiling arcane energy in the form of blue flames that destroyed Dweomerheart and continued to spread across the multiverse.
Anything that came into contact with the Spellplague risked being destroyed or warped by its energies. Creatures and individuals risked becoming "plaguechanged", which entailed massive physical mutations and often a complete loss of sanity, and sometimes bestowed the creature or individual with terrible powers. Those who came into contact with weaker versions of the Spellplague were sometimes lucky enough to escape with only a spellscar, which could also bestow potent magical abilities. Physical places could also be horribly warped by the Spellplague, becoming a changeland.