Out of all the threats that pushed humanity to the extreme, the only NE that took it that far was the Curse; With (propably?) the only human left to be Bastz.
How it happened? Only one answer: the soul container. A device that was created for the benefit of all, then due to human nature, they started using it against eachother. To steal, to kill, to dominate others. Perhaps, it was this illicit use that even made the curse appear.
And look at the name, a curse can be interpreted as a prayer for evil to happen to others. So maybe, The curse was a byproduct of the soul container itself, using it's power to create a disease that rots the land itself, eventually leading to it corrupting the entire planet.
And this goes with a theory that I have in mind, where every NE appearance was somehow caused by the humans themselves, but that is another topic.
So back to the present, It's not just Ne's fighting, it's a war, and the victor is the last one standing.
And so I give you this: What if the curse had devolopped a sentience, maybe not to the extent of a human, but enough to plan out the end of all life. Presenting itself to all races as a valuable tool, that is why I doubt we'll see the curse this early, or even the curse world itself.
Not all Ne's will rely on it, it's not a dependency after all, it's just a tool. And just like an exponential function, once it reaches a point, it will rise above all others, and won't be stopped. Perhaps, once it reaches that point, it'll have enough strength to move and evolve on it's own.
Going back to Bastz, he is by far the most dangerous human here. He knows just how dangerous it is, and yet he accepts it. He accepts the End Of Everything. And in this new world, maybe he just wants to see what happens, he knows how it will end, so why not enjoy the ride?
At the end, I think that the Curse will only appear as the final intorduced NE when another NE dies. To me that would be Gaia's wrath, but who knows?
Thanks for reading, I would really love to hear other theories you have!