Spellplague

In 1385 DR, the ascended deity Cyric, aided by Shar, murdered Mystra, the goddess of magic, in her domain of Dweomerheart. This act ripped asunder the fabric of magic in the world, unleashing its raw power in a catastrophe called the Spellplague. Thousands of practitioners of the Art were driven mad or killed, while the face of Faerûn was reshaped by waves and veils of mystic blue fire. Entire nations were displaced or exchanged with realms from other worlds, and parts of the earth were torn free to float in the air.