The Cataclysm is the defining event of the new world. The culmination of stupid choices, ignorance, extraplanar magics, and a dragon. When it was all said and done, the Cataclysm wiped the new world of more then eighty percent of it's population.
The cataclysm was triggered by the dragon Valinthax as a way to reset the universe and stop it's impending collapse. But in doing so, a rift was opened and fire rained down from the container dimension that all universes exist in. This was necessary to purge the destabilizing energies from the universe, and they had to be replaced with something to maintain the balance.
Basically neutral to any universe, the energy that entered had one unfortunate effect. As it entered the universe it crystallized, having to conform to the physical laws of the reality. These crystals, when they impacted the ground, created an energy field that would suck the souls out of those in proximity to the crystal. What remained were nothing but shambling husks, left to the collective will of the spirits locked in the crystals.
The crystal zombies started to drag those not killed by the crystals to them. One touch would cause the persons soul to be absorbed into the crystal. In a short time, the major cities and many of the larger towns were nothing but cinders and the walking dead. This is the cataclysm, and it freed the new world from the grasp from the old one. It may be hell...but at least people are free to be what they want.