I'm going to post this in a (hopefully) similar manner as some of the rest of your mythical material, but it's also a first draft. Feel free to change/improve/ignore/etc it, or ask questions (for which I may not have answers
).
Although most who do so are driven to utter madness or suicide, enough have tried to scry upon the beginning of the universe that a patchwork history of that ancient genesis can be put together. Trying to scry upon the far distant future reveals nothing at all, and is even more likely to result in madness or death.
In the beginning there was only the yawning emptiness of the void; it's black expanse so unfathomable that even to gaze upon it is enough to crush weaker men's souls. Then in a sudden rush, all of existence suddenly burst forth from the nothingness. It was in a raw state, and not recognizable, but all of the components for what now make up the world, stars, and even the gods and souls of men were already present and after some indeterminably long time began to coalesce into now-familiar forms.
However, the void had not been a mindless emptiness. As these new primal forms and energies poured forth, the void's hunger was now revealed. It longed to consume these new things, drawing them back into the nothingness of itself, and snuff out all until nothing was left but itself again. By now, the first gods had arisen however, and they of course did not wish to be consumed. And so they used their own powers to divide the cosmos, sacrificing some of it to the void, but hiding other parts so that it (and they) might persevere.
And so it remained for eons beyond reckoning, and the void itself was forgotten to even those who still survived and had once fled from it. And then Lledrith, in his lust for power, tapped (either knowingly or unknowingly) into the Void, in order to create the Undead. The Undead are all literal conduits to the Hunger of the Void, and are fueled by drawing the energies of creation through themselves and into the void. Even the briefest passage of such raw energies is enough to animate them, and indeed
ancient undead become so coupled to this flow that their wounds knit even faster than the most vigorous among the living.
And so it was that the Void was awakened again to the presence of Creation, and began to hunger after it, and once again pursue it. And now all of existence is in jeopardy, unless all of Creation can unite in their efforts to stave off or flee and hide from the Void's approach.
Basically the idea is: what if the creator wasn't benevolent? What if genesis might not have even been intentional? What if the act itself (involuntary or otherwise) had left the creator in a weakened state, and the creator wanted nothing more than to reclaim what was lost from it in the act (and thereby undo all of creation)?
Perhaps there are only a limited number of souls available (all of which sprang forth in the same instant of Creation/Genesis as everything else). If reincarnation is a thing, then I guess Drancedigaeth let's them leave his realm. If not, then they'll eventually be a dwindling supply, which might also explain the drop in population (maybe there are increasing numbers of stillbirths as well as plagues, etc?). Also, the souls of the undead are slowly consumed by the Void and once they are finally destroyed pass on and cannot be brought back by any magic (like those consumed by barghests, which should probably also be related to the Void somehow).