Indeed, it's tough to suss out. I'm still digging through it, and it's even
further complicated by [1] that the denizens of the First would - not even knowing what voidsent are, as evidenced by the caster arc - would not have other words than "transformed" / "transmutated' ... and by [2] that all of these events were truly conspired by the Shadowkeeper ... who was
herself from the Thirteenth,. The Shadowkeeper passing on her Ascian masters' knowledge of magicks and voidpacts (as Lahabrea's minions did in the Source many times) is not a stretch of the imagination.
If that is indeed the case, we're back to the monsters we see being pre-existing voidsent, whereas sin eaters are yet to exist, and thus we're back at square one.
The risks of not working from a "facts have citations, otherwise it's theory" mindset.
Y'shtola for lore forum mascot. /s