It's as in real world - start with people forgotten by society, offering them acceptance, dumb and uneducated by simply brainwashing them. Buy some others with promise of power.
Also, Twilight Hammer and old gods were mentioned. I haven't finished questing yet, but the black locus thing looked very old-godly. And Old gods always had a connection to elements, plus hated Titans. So my expectation is that it will turn out old gods / void lords are behind everything in this expansion (considering Raszageth was pulled by a void portal created by deathwing and we don't know what happened with her between then and her imprisonment.
And probably, some primalists are ex-twilight hammer (they always reminded me of twilight hammer in silithus with their storm / lightning theme and purple robes
Again, I have not finished questing yet so no idea if there's something later on which would debunk that