How it all started
Cataclysm
Many humans will remember the Cataclysm as a singular event, when the dead came to life and unimaginable horrors took to the streets to pour death and carnage onto the Earth. However, it was actually a fairly gradual process. Dimensional research, both by the United States and various other nations, slowly weakened the dimensional walls separating our reality from the Nethereum, at least on Earth. The effect of this was fairly gradual at first, with mutations becoming easier over time, revivication near portals becoming faster and stronger, so slowly over the years that few even noticed.
The tipping point however, was when XEDRA decided to open one of the largest portals of ever devised by their minds in New England. This weakened the dimensional walls enough for it to shatter, letting a deluge of both XE037 and it’s associated energies into our reality. The proceeding event is what is now known as the Cataclysm, as horrific portal storms and all the worst nightmares of a few dozen realities flooded onto our world for several days. By the time the tides settled, the Nethereum and ours finally reaching an equilibrium, the world had been devastated, governments had collapsed and civilization as we knew it was destroyed.
The various portals throughout the world keep ours and the Nethereum connected, but this is not a truly stable equilibrium. If the walls between our dimensions are not repaired, if not enough portals are closed or stabilized, eventually the Earth will suffer a local dimensional collapse, folding into itself like a used bandage to quarantine us and protect both the rest of our reality and the Nethereum from further dimensional destruction. (Timeline unknown because if it’s within a lifetime I guarantee you some player will wait until that date just to make a bug report that nothing happened.)