The letter you brought me was written by a being who calls himself Nagaz, a member of the Argus Wake. I don't know what this Argus Wake is, but if they're in alliance with the Syndicate then they must be a threat.
I want you to find Nagaz and the Argus Shadow Mages mentioned in that letter. I want you to find them, and kill them.
Search in the Uplands, where the Syndicate is strongest. It is north of Strahnbrad.
With luck, you'll find these envoys of the Argus Wake there.
Well done, <name>. The Argus Wake does not sound like a group we want running the Syndicate.
And although you may have destroyed their leader here, Nagaz was merely the servant of another. The Argus Wake might run deep through the continent of Azeroth.