This quest was marked obsolete by Blizzard and cannot be obtained or completed.
Put on your Recruit's Robe, go to the Jaggedswine Farm and get the Book of Incantations from Overseer Jintak. Read from the Book of Incantations near the Cultist Altar to practice the ritual.
The cultists seem assured of their victory, although we have captured many of them. We must find out more about the ritual mentioned by the cultist you questioned.
These rituals must be the core of their plans to attack Orgrimmar.
Take one of the robes from the arrested cultists; it's not like they're going to need them anymore. Put it on and go back to the cultist outpost at the Jaggedswine Farm in Durotar and see what you can learn about these rituals and their place in the cult's plans.