Meet Karam Magespear at Felbreach Hollow in the Broken Shore.
Though I have never heard of this "Nethersworn" cult, what this manifesto speaks of can't be good.
I do have a theory, however. The writing speaks of "the place its skin is thinnest." I think they might be referring to the place where the veil between Azeroth and the Twisting Nether is weakest - the Tomb of Sargeras.
With our forces having established a foothold at the Broken Shore, this would be an opportune time for Raest and his cultist allies to reach the tomb. I would suggest you stop them.