Rodorsceaft: Pillar of Heaven
The Withering was a massive environmental disaster that took place a few hundred years prior to the Demon War. It was precipitated by the study of Defiler magic by the wizards of the kingdom of Moros.
The facts of the Withering are, at present, largely uncertain. What is definitively known is that Moros was host to many powerful defiler wizards, enough to challenge the Draiocht. One day, the Day of Withering, they enacted a powerful Defiler ritual that drained all the life from one of the branches of Mother Tree.
The Withering had several important consequences which affect the world even to this day. The first was the creation of the Barrens, a vast, desolate area completely devoid of all life, centered around where Moros once stood. The second was the weakening of the Defilers, who were then hunted to absolute extinction by the Wild Reapers. The third was the Geas Dreodor, whereby the Draiocht outlawed the practice of wizardry everywhere in the world, on pain of death.