I just spent 20 hours sitting in Southshore waiting for Shadowy Assassins to spawn to get the Assassins’s Contract. Nothing.
I still don’t know what spawns them (completing a quest like Noble death or server timed) but it seems to not spawn at all. People are questing and many people are turning quests to Magistrate Maleb. Southshore was busy with people going in and out all day and night, yet nothing spawned.