The Spellplague was a period of time when magic grew chaotic and random leading nearly to the destruction of magic as it is known. The cause insofar as it is known was the tainted touch of the Crippled God bleeding into the Astral plane and ended with the Chaining.
History of Scadrial
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922 BS
926 BS
Spellplague
Plague / Epidemic -
926 BS
926 BS
The Chaining
Metaphysical / Paranormal eventA gathering of Ascendants and gods joined together to bind the Crippled God to the world and then excised the area from the it. Quarantining him and his miasma of chaos into it's own demiplane. Many ascendants and gods were shatterd, scarred, or otherwise changed from this process.