Spellplague

The Spellplague was a malady that struck Realmspace on 29th Tarsakh, 1385 DR and was caused by Mystra's assassination at the hands of Cyric and Shar. With the goddess's death, the Weave, the universal structure of arcane forces, convulsed. It continued for a decade, leading to the Wailing Years, during which time arcane magic ceased to function and the planet of Toril was transformed.

""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 Undead by Richard Lee Byers

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, is the assassin, Savras, the god of divination, is the soothsayer, and the city, cavern and tree symbolize the ordered structures of magic crumbling into chaos.

The break-down of the Weave could be felt by all wizards across Faerûn.

Storm
The storm began over the Mhair Jungle in Halruaa and quickly grew to a large size, blue flames visible in the sky. It laid waste to Halruaa due to the heavy wild magic activity in the area, and Sespech, the Golden Plains and the Nagalands are transformed into an unpredictable and surreal place, dubbed the Plaguewrought Lands. Virtually none of Faerûn was unaffected by the Spellplague and certain areas were eliminated entirely, while others were created anew. Thousands of mages were either destroyed or went insane due to the collapse of the Weave after Mystra's death. In Cormyr, a third of the War Wizards were either killed or driven mad. Those who survived were forced to learn martial combat as a result of the disappearance of arcane magic.

Effects on arcane magic

 * With the structure of magic out of balance, even with great effort many spells failed or produced unreliable results.


 * Most permanent magical items, such as artifacts, were left intact at the end of the Spellplague and charged magical items were either destroyed, warped, or simply ceased to function. Items with dormant magical qualities, which were manually activated, were largely unaffected by the Spellplague.

Other effects

 * The Spellplague affected the landscape, making the ground rumble and heave up and down like the surface of the sea. Curtains of blue flame swept the landscape, reshaping the land by cutting crevasses or lifting and sculpting the plain into hills and ridges.


 * Waves of azure fires appeared everywhere, killing whatever they touched.


 * A surge in the use of technology, divine magic, and worship of the Deities of Knowledge and Invention.


 * Political upheaval across the lands that resulted in many attempted coups, invasions, and uprisings. Especially against governments, and nations that had relied heavily on arcane magic for defense or controlling their population.


 * The god Azuth was thrown from Mystra's home plane, Dweomerheart, to the Nine Hells, where Asmodeus stole his divine essence and became a god.


 * Parts of Abeir and Toril switched planes, swapping Maztica and the continent of Laerakond, which is now known as "Returned Abeir."


 * Mulhorand was completely destroyed, and the Mulhorandi pantheon disappeared. The land was later settled by Deep Imaskari and became the empire of High Imaskar.


 * Unther was obliterated when a fragment of the dragonborn nation of Tymanchebar was transplanted from Abeir onto it, eventually becoming the realm of Tymanther.


 * Chult became a large island.


 * Halruaa was destroyed and turned into a land full of rampant spellplague.


 * The Shaar became a wasteland called the Shaar Desolation.


 * The region surrounding the Great Rift collapsed into the Underdark, creating an enormous cataract in the earth called the Underchasm.


 * The halfling realm of Luiren was completely submerged underwater, becoming the Gulf of Luiren in the Great Sea.


 * Lantan was hit by massive waves and flooded, killing all of its inhabitants. However the nation had resource camps in other locations, namely Chult, so not all Lantanese citizens and technology may have perished along with the actual island.


 * The peninsula known as Var the Golden becomes a region of the Great Sea called Var the Drowned.

Aftermath
By 1395 DR, the majority of the effects of the Spellplague had come to an end and most arcane magic had returned to normal.

By 1479 DR areas of Toril still affected by the Spellplague were referred to as Plaguelands.