What is exactly the "void" and where does it come from?

[Zojha]

Well, for context, it should be noted that Eorzeans class everything "alien" as "voidsent" until further notice. And since the lore team apparently likes to misguide people in an attempt to make the world naming more realistic (even if it comes at the cost of regular inconsistency), it can well be there are misnomers about and not everything named "Void-" is actually related to the void.

That said: From what we know, "the void" is the 13th reflection of our world, which was consumed by darkness and thus failed to be rejoined with the source world (ours). In that context, Nabriales calling upon the "insatiable hunger of the void" to "return the world to perfect darkness" in the crysalis begs a little question on the sincerity of his rejoining endeavours >_> But oh well.

I wager our lore masters can explain it much better, but...yeah. That's the gist, I think.