Spellplague

The Spellplague, called the Blue Breath of Change by the inhabitants of Abeir, was a disaster that struck Realmspace and even the planes themselves on the date of Tarsakh 29 in the Year of Blue Fire, 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 arcane magic ceased to function and the planet of Toril was transformed.