We had warning that this was coming. When Ragefire Chasm was given an overhaul in Mists of Pandaria, the changes didn't escape the notice of players -- but nobody really knew the extent of what was going on. Remnants of the Twilight Cult from Cataclysm could be found in the depths of Ragefire, carrying insignias that noted they were part of a new order -- not Twilight Cult, but something quite possibly far darker, called the Dark Shaman.
These shaman were highlighted briefly in the novel Tides of War, as part of Garrosh's assault forces on Northwatch Hold. While the question of how they came to be seems to be fairly self-explanatory, there's a little more to the story than previously thought. It stretches all the way back to the days of Wrath of the Lich King, and the discovery of a different, new, hardy race of warriors and shaman that were far more used to doing what was necessary to survive, than what may or may not have been right.