Day of Mourning
There are as many theories to the origins of the Day of Mourning as there are souls in Khorvaire. What happened on the Day of Mourning? There are many theories on what happened that fatal day and no one knows for sure.
Some believe that an Eldritch device planted by the Royal Eyes of Aundair caused the heavens to open up destroy Cyre. Others believe the Cannith creation forges were overworked, leaking uncontrolled arcane engery causing the devestation that resulted in the Day of Mourning.