Geological / environmental event
The Triarch went to far it is said. The ritual razing the land. It was arid, now it is dead.
"The Triarch wished to warp the land in their favour, making the crops and food of the Drakekin naught but dust. His power and might were relatively untested at the time but in what happened to Elgas it is clear the Triarch is the most powerful mage alive." - Chronicles of the Elgasian war part 14 The Blasting of Elgas, Cerrin Huit