Spellplague

The Spellplague is a malady that struck Faerûn on 29th Tarsakh, 1385 DR and was caused by Mystra's assassination at the hands of Cyric and Shar. 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.

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.

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 effected by the Spellplague are referred to as Plaguelands.