The Spellplague

The Spellplague 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.

Effects on the Dungeonscape

Though undeniably disastrous, the decade-long effects of the Spellplague and Wailing Years were limited in the Dungeonscape. The corrupting Storm of Blue Fire that ravaged the Prime Material world of Toril never appeared in the dimension, preventing the appearance of Plaguelands and other effects. In the century since, cosmologists believe this to be due to a temporary closing of the gates between the Dungeonscape and the rest of Realmspace as the multiverse rearranged itself after the death of the World Tree.
Despite this, the loss of arcane magic severely affected portions of Tiriande. A great deal of wizards and sorcerers went mad, some outright disintegrated by attempts to channel the Weave and others merely unable to do so. The Royal University was forced to close its doors, though some of its mundane courses persisted in other locations. 
Further disaster was prevented through rapid response by the Spirewardens and Covenant in cooperation with the churches of several other deities like Lathander and Deneir, whose divine magics supplemented the city's defenses until the end of the Wailing Years by 1395 DR.

Effects on the Prime Material Plane

Mystra's death caused the Weave to collapse and, without the Weave, the Shadow Weave was unable to be maintained and collapsed as well. The breakdown of the Weave was felt by all wizards across Faerûn. The corrupted madness of Cyric defiled what arcane forces remained from the dissolution of the Weave, resulting in a new magical source of defiling arcane energy in the form of blue flames that destroyed Dweomerheart and continued to spread across the multiverse.

Date of Setting

1385 DR