The Great Cataclysm is an event that happened centuries ago and formed the Sundered Lands to be the hostile lands everyone knows today.
It was a massive volcano eruption that purged an entire region of life (currently Ashen wastes), leaving not many creatures alive, that then adapted and survived - turning into monsters.
A warlord called Vorathul Nox-Kael was once feared across the lands - bringing death and destruction with his warband wherever he went. When him and his band were finally hunted down and all but wiped out, he was the sole survivor - barely escaping with his life and vowing to bring death and ruin to everyone who stood against him. He has succumbed to his injuries soon after the encounter but has refused to stay dead.
From the afterlife he whispered promises of great power to ambitious cultists, who brought him back to life through a ritual - and were promptly slain by him. With newfound life Vorathul Nox-Kael enacted his revenge plan by raising an army of undead warriors and used thousands of souls to fuel a ritual at the present Black Maw that erupted the Volcano with Cataclysmic strength.
The Ashen wastes were born and grasslands to the south were turned to the Bakhri Desert that is present to this day. The Ashen wastes turned into the survival of the fittest, where the weak died and the strong evolved and turned to monsters wandering those lands to this day.