Demons reform in The Twisting Nether which I guess is the fel lands, if they are permakilled there I think they are just gone.
Shadowlands is more than 4 domains, Helya's is one of the others.
"The First Ones"... Yeah...
All this stuff was poorly explained.
Its a whole clusterfuck of lore and this is how Ive read into it. The progenitors dont really have a specified origin. They created all that we know of within WoW. Where as the First Ones, upon playing the story, would make you feel like they are all robots. Which people indirectly fill in the origin of the other Eternal Ones with Zovaal's origin. But Zovaal was the Arbiter and he was "created" as an impartial servant to judge souls. Zovaals character arc was that when he read the souls of those that died, from different universes etc. He found that "something" was coming that would ultimately destroy the balance. So of course we kill him and put Blue Fag in as the new Arbiter and disregard his ominous threat. (keep in mind that these are just "roles" and doesnt define what they are. Anima? A Robot? A stone statue (dwarves, gnomes, etc.)?
My theory and... What no one seems to read into is that blue fag is a Kyrian, which is based on the premise of Order. Which I think is playing right into what we've been grasping at since Legion (Burning Legion - Disorder). With Order so disproportionately in favor of the Arbiter, we can expect that the balance between the two to be disastrous, leading to the tilting of the entire balance of the Universe in favor of the Light. (Remember that Void and Light were around before the others.) (Nevermind the death of Denathrius (Disorder in SL's Realms) and the fall of Revendreth.)
So our future Big Bad, which may or may not be in this expansion (DF) is "Light." After we beat light, we'll have some little lame shit happen where we dont outright exterminate Light because we need to keep the balance.
I think the premise of all this shit is interesting, but the way its being presented is just God awful.