Spellplague

The Spellplague was a disaster that struck Realmspace on 29th Tarsakh, the Year of Blue Fire, 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 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, and the city, cavern and tree symbolized the ordered structures of magic crumbling into chaos.

The breakdown of the Weave could be felt by all wizards across Faerûn.

Storm
The storm began over the Mhair Jungles west of Halruaa and quickly grew to a massive size, with 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 were transformed into an unpredictable and surreal place, dubbed the Plaguewrought Lands. Almost every part of Faerûn was affected 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

 * Despite the efforts, with the structure of magic out of balance, many spells failed or produced unreliable results.


 * Most permanent magic items, such as artifacts, were left intact at the end of the Spellplague but 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.


 * The Wards of Silverymoon were shattered.

Effects on the Gods

 * Savras died as a result of the dissolution of Mystra's home plane, Dweomerheart.


 * Azuth was thrown from Dweomerheart to the Nine Hells, where Asmodeus stole his divine essence and became a god. Dweomerheart itself was dissolved.


 * Cyric was tried by a conclave formed of Lathander, Sune and Tyr. They found him guilty of Midnight's murder and sentenced him to imprisonment in the Supreme Throne.


 * Lathander transformed back into Amaunator.


 * Nobanion tried to save a pride of lions in Gulthandor from oncoming blue fire. He came in contact with the blue fire, and it mutated him into a dark and bestial lion with a mane of blue flame.

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. Shards of earth wrenched themselves free and became earthmotes


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


 * Rifts opened in the Underdark beneath the Sea of Fallen Stars, causing the level of water to drop, cutting many ports off from their livelihood.


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


 * Political upheaval across the land 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.


 * Large parts of Abeir and Toril switched places (small, almost imperceptible parts had been switching places since the Time of Troubles but this event was markedly more noticeable), swapping Maztica and the continent of Laerakond, which became 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.


 * Southeast Chessenta was also obliterated by the appearance of Tymanchebar. Other parts of the nation collapsed into a rift into the Underdark. Unlike Unther however, Chessenta survived and prospered - though it's beloved ruler Tchazzar disappeared without a trace.


 * Chult became a large island when most of the Chultan Peninsula was cut off from the mainland. Much of Samarach was drowned, it's survivors spellscarred, and the yuan-ti realm of Serpentes fell. The city of Mezro collapsed and its ruins became occupied by the undead remains of those who didn't make it out in time. Ubtao's barae were lost and the jungle became home to massive monsters from Abeir.


 * Evermeet was pushed into the plane of Faerie, leaving behind a pale shadow of what the island of the elves used to be, though the isle's residents could move back and forth from Faerie to Faerûn at will.


 * The island of Nimbral completely disappeared.


 * 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. Bits of what used to be the Underdark began to float above the Underchasm as earthmotes.


 * 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 became a region of the Great Sea called Var the Drowned.


 * The Misty Vale was devastated by blue fire, though the wild elves that occupied it survived. Known as Elfharrow, it was fiercely defended from all intrusions.


 * The illusions that cloaked the settlements on Selûne failed, revealing them to those on Toril.

Aftermath
By 1395 DR, the majority of the effects of the Spellplague had come to an end and most arcane magic had returned to a semblance of normality, with a handful of major changes to the way spells are cast.

The Empire of Netheril came across the Lost Vale and displaced the saurials there into the Underdark.

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

By 1479 DR the Spellplague was ended with the event known as Mystra's Return. The Weave was resurrected with the goddess and magic became what it was before the Spellplague, though some areas with lingering effects from it remain.