Spellplague

The Spellplague is 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.

Source Zauberpest