the world that existed before the calamity was the Old world;
Calamity: "After the Calamity not a single mortal human was left alive in the Old World.
The Calamity sundered the very reality of the world, splitting it into infinite alternated dimensions."
Then the calamity happened and created a bunch of copies, they each have a scar from the calamity, but the one on the old world was the worst.
I also remember from the old campaign they mentioned that the calamity affected the different worlds to different levels. I think it was avarrach that mentioned that almost no humans there died, and they remembered the calamity and so outlawed magic.
So maybe they were further away from the calamity in a sense, and the Old world had all mortal humans killed, that might also be why it is considered the epicentre of the calamity.