The cultists seem assured of their victory, even as we detain more and more of them. We must find out more about the ritual mentioned by the cultist you questioned.
These rituals seem to be the core of the cult's plan to attack Stormwind. I'm going to give you one of the robes taken from the arrested cultists. Put it on and go back to the cultist outpost at Mirror Lake Orchard in Elwynn Forest and see if you can get the cultists there to let you in on the plan.