It comes from the depths of the Darkhold

[Fernehalwes]

The Void encompasses everything beyond the realm we can comprehend. I assume The Void is a catch-all term for anything that doesn't share our senses of reality and/or lives in different dimensional space. Therefore, it's reasonable that the mobs we see coming through are just those from a certain area of The Void where things aren't overly dissimilar from where we dwell.

Inside the Darkhold, a rift has been opened between the worlds. Batraal entered and slaughtered everyone working there, and all of the other mobs flooded through as well. In this case, it's Atomos that has linked the realms and is flooding it with enemies, but theoretically they're from a relatively similar space as those that now in habit the Darkhold.

Much like how hippogryphs were able to find their way out of the Void into Eorzea, the ogres have as well. This time, wherever the Deepvoid Slaves come from is where the Atomos pulled from, I guess.

Preach it, Anonymoose!

Or they just felt like copypasta was a good idea.

Hey...