The Spellplague

The Spellplague is a malady that struck Realmspace on 29th Tarsakh, 1385 DR (96 years ago) 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.
 * "The white queen is troubled but can't say why. The black queen hates the white and gives the assassin a black coat. The assassin steals upon the white queen. She can't see him gliding through the shadows. The sword screams. The white queen falls. Her city falls. Stones fall in the cavern to crush the soothsayer. The tree burns and thrashes in agony. Branches break. Branches twist and grow together..." --- From a vision by the diviner Yaphyll.

In her vision, Yaphyll refered to Mystra as the white queen and to Shar, goddess of the night, as the black queen. Cyric, god of murder, is the assassin, Savras, the god of divination, is the soothsayer, and the city, cavern and tree symbolize the ordered structures of magic crumbling into chaos. The break-down of the Weave could be felt by all wizards across Faerûn.

The 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.

On magic
With the magical structure being out of balance, any attempted spells failed or produced unreliable results. Only with great effort spells came out right as often as not. Permanently magical items such as artifacts were, by and large, left intact at the end of the Spellplague though most charged items were either destroyed, warped into something else, or simply ceased functioning. Items with "dormant" magical qualities that were activated manually were, however, largely unaffected by the Spellplague entirely.

On the lands
The Spellplague affected the landscape, making the ground rumble and heave up and down like the surface of the sea. Curtains of blue flame swept the landscape reshaping the land by cutting crevasses or lifting and sculpting the plain into hills and ridges. Waves of azure fires appeared everywhere, killing everything it touched.

The aftermath
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 affected by the Spellplague are referred to as Plaguelands.