Idle thought: how many of those masked men have we seen? They're the Ascians, right? Could they be the lords of the Seven Hells? I suppose I'll have to count more actively once I've seen the whole story in order to form a more educated guess, although like anything else we can't speak with certainty unless it's made abundantly clear with in-game text.

This reminds me once more of the Dragonriders of Pern. The dragons cross into a place of extreme cold and sensory deprivation called "between" when they teleport. They can even time travel that way. The only differences is that they can only do so with a conscious effort, and it never happens without their triggering it intentionally. They also can't travel to coordinates that don't exist, and will only wind up in places that best match their mental images. They can even teleport to points that are occupied by other objects, such as a solid mountain, resulting in immediate death.

The authors never depict a dragon crossing into other realities, although there was a short story where they suggest a realm "beyond" that I have yet to read. I'm curious if it's simply a final resting place for the dead (which it seems to be) or if there might be other realms that could be reached if only a dragon had a guiding image. I'm also still not sure if between itself is actually a place, or if it's simply what we experience when traveling in a manner beyond what we can perceive of physics.

Back On Topic
I wonder how much we could learn from a clearer understanding of Atomos and the "Voidsent". The word itself could just mean "thing from elsewhere", which would allow for the notion that Atomos can draw monsters from various realities and throw them up on Hydaelyn. If I remember correctly, anything in the "Voidsent" category was coming from those gaping maws. Side Note: How many aggressive maws were there, exactly? 2 in Mor Dhona, 2 in Thanalan, 3 in Coerthas... how many in Shroud? I'm probably overthinking this.

No, it's most likely that Atomos' mouth leads to one place, and because anything "Voidsent" can come from Atomos, then anything "Voidsent" comes from that one place. It's clearly not so chaotic that it can't support cohesive entities like the Deepvoid Slave or Fernehalwes, which can exist in our world under our laws of physics. Alternatively, maybe Atomos is a biogenitor like Titan in FFXIII, creating the Voidsent himself. Does that mean the Ascians can do the same? Wasn't the BLM final boss something like that? I suppose that would break tradition, since he's usually an inter-dimensional portal.