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 Gartok Ragefist at the Ring of Valor in Orgrimmar's Valley of Honor. After you've returned with your prisoners, question the Detained Cultist.
The note you found can only mean one thing. The warchief's proclamation was 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 of them, but we have to get as many as we can. Use these cuffs to arrest the infiltrators and then deliver them to Warden Gartok Ragefist at the Ring of Valor in the Valley of Honor.