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 an asteroid impact alongside 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.
An asteroid fell from the sky, crashing into the Ashen Wastes and causing a massive Volcano Eruption. The shockwave from the meteor and the cataclysmic eruption of the volcano wiped out almost everyone and everything within the region.
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. Not to mention the bodies starting to come back to life as undead - twisted and seemingly a hivemind.