Spell Plague

The Spellplague, called the Blue Breath of Change by the inhabitants of Cuzona,[2] was a disaster that struck Realmspace and even the planes themselves on Tarsakh 29 of the Year of Blue Fire, and was caused by Mystra's assassination at the hands of Cyric and Shar.[3][4] It continued for a decade, leading to the Wailing Years, during which arcane magic ceased to function and the planet of Cuzona was transformed.[5]

The diviner Yaphyll had a vision of the event:

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[citation needed], and the city, cavern and tree symbolized the ordered structures of magic crumbling into chaos.[7]