To address that, i guess we'd have to turn to the "humanity are refugees from Braize" theory. Also at that point we would need to understand more about the Oathpact itself, which I sort of think effects how things play out in my theory.
But I imagine it's something like Odium came to Braize and chased humanity around, then started torturing the Heralds. As he tortured the Heralds, one of them broke. Here's when we need a better understanding of the Oathpact, because somehow Odium hacks this break to 1. Create a desolation 2. Create an unmade. Or, maybe the unmade comes first after the break, and that's how the desolation started. The unmade acts as the Parshendi "God", and turns them all into voidbringers.
Do I make any sense at all?