This quest was marked obsolete by Blizzard and cannot be obtained or completed.
Use the Heavy Manacles to arrest 5 Cultist Infiltrators and bring them to Warden Thelwater at the Stockade. After you've returned with your prisoners, question one of the Detained Cultists in the Stockade.
You were right to bring this note to me, <name>. The king's proclamation came too late; they already have infiltrators at work in the city.
This note contains instructions and locations for cultists to meet, but it doesn't say what they're doing. Our enemies have grown smarter. It's going to be tough to track down all the infiltrators, but we have to get as many as we can. Use these cuffs to restrain any agents you find, bring them to the entrance of the Stockade, and report to me when you're finished.
Excellent work, <name>. The note you captured makes sense in light of this ritual the infiltrators mentioned. This city is infested with cultists and their sympathizers. We must act before their plans can come to fruition.