This quest is not available in game.
Go to Orgrimmar's Drag and speak to a Doomsayer or take a robe from the crate near a Doomsayer to join the cult, then put on your Recruit's Robe.
The streets of Orgrimmar are wild with rumors about the cult's origins and practices. There's only one way to know the truth about this group, <name>. You're going to join them.
Approach one of the Doomsayers in the Drag and tell them you wish to join their group. With any luck, they will give you instructions to introduce yourself to one of the group's leaders and tell you where the group is hiding. Don any clothing given to you by the doomsayer; you'll need it to gain entry to the group's hideout.