This quest is not available in game.
Speak with King Varian Wrynn in Stormwind Keep.
You have successfully defended Stormwind from the threat posed by this Twilight's Hammer cult, <name>, and without harming our own citizens. Our enemy will not rest for long. Indeed, Cho'Gall must already be busy planning his next move.
You must take word to King Varian Wrynn. Go directly to him in the throne room of Stormwind Keep. Tell the king that our enemies have shown themselves and we must be prepared for their return.