Revelations from the Announcement Showcase

[Zohar_Lahar]

The alternative explanation is that the Voidsent are beings corrupted by darkness. We see this process replicated in the tank role quests. We also know the beasts of Terminus were the product of creation magicks and the dread fears of the Amaurotines, after the sound triggered the loss of control over their powers. The DNC quests also have a similar phenomenon where anxiety yields creatures of nightmarish aspect, but in this case something emanating from the earth (in which the Underworld aka the Aetherial Sea is to be found) in the form of a sound preceded the phenomenon and then led to loss of control over their powers. My guess is darkness has a lot to do with an active state of aether, and any heightened emotional state may be more prone to yielding it (also suggested in the DRK lore I believe), thus the form is more the same end product but from a different process, created from uncontrolled creation magicks impelled by dread. The 13th only became that way after Igeyorhm rushed a calamity there without the Source being in a "receiving" mode at the time, when the Ascians were still figuring out the whole process. So I do think there's a connection but specifically in the process (overload of darkness) that yields such "demonic" forms.

It wasn't just creation magicks going haywire; "The sound distorts all living things within earshot". Not unlike what the Flood of Darkness and Flood of Light has demonstrated.

Theoretically it's possible, but given several Sin Eaters were also reskinned voidsent enemies, I'm not putting stock in the idea.

If we operate under the assumption Emet-Selch was telling the truth about what happened to the Thirteenth, and in good faith that he never lies...

How would that contradict what Emet-Selch said about the Thirteenth shard? His Amaurot recreation made it impossible not see a connection between the Final Days and the Flood of Darkness/Light.