The Spellplague

In the Year of Blue Fire (1385 DR), a magical disaster called the Spellplague changed the face of Toril, its lost sibling Abeir, and even the plan themselves. Most scholars believe that the Spellplague was caused by the muder of the goddess Mystra (godess of magic) at the hands of Cyric. Whatever the cause, the Spellplague wrought huge devastation on the Realms.

Ancient realms that had passed beyond easy reach of the world were pulled back, such as the Feywild and the Abyss. The long-forgotten sibling world of Abeir also burned in the Spellplague, and some areas (including an entire continent) of Abeir passed into Toril.

Today, the Spellplague is a memory most commonly seen by some weird geographic features in the Realms. Floating islands called Earthmotes hover over the ground, floating along on air currents. Portals between far-off areas sprung into existence, but few of these have been mapped. Some rocks and plants glow. Passages to the underdark appeared, as well as entrances into the feywild.

There is currently no remnant effect on magic from the Spellplague. The Weave has been reestablished, and all magic functions as it did prior.
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Dungeon Master - The Dark Campaign
Sir Muurak - Mul Cavalier 14, Scales of War
Demuriath - Revenant Assassin 5, Points of Light
Kurvilis - Gnome Mage 4, Eberron