The Celess'Dur Calamity

Lead-up
Between 104-131 EA Celess'Dur expanded its borders northwards, conquering much of the Wildlands and crushing various primitive tribes. Following this, Potentate Ereviros determined that the forested Wildlands must be made habitable and productive in the same ways that Meslona's Cradle was. In preparation for this, he formulated a ritual whereby hundreds of mages could commune at once.

The Event
In 157 EA, Potentate Ereviros prepared his ritual fully, and approximately 3000 mages of the Magisterium entered into the ritual, pooling their powers into a single spell. Most of the mages had been told that this would be a natural magic, to cause the forests to recede. However, Ereviros, who was commanding the arcana, realised halfway through the ritual that this was not possible. Rather than stop and be unable to carry on at all, Ereviros changed the spell into a blunt instrument of destruction, and created an enormous wall of flame. This flame encompassed the entire barrier of the southern wildlands, consuming a vast amount of the forest.

Aftermath
Approximately 1400 of the 3000 mages died of arcana fatigue, something that Ereviros had not predicted as a result of the spell. Ereviros was deposed as Potentate in 159 EA. Thousands, if not millions more people were killed in the inferno that resulted in the Wildlands. The fire died when the spell stopped, leaving a barren ashland in its wake. After a few decades, this land did become fully habitable, thanks to nature magic aiding in the regrowth of the land into a fertile area for agriculture.