The masses, especially the 'great unwashed masses': the general population, or (yet more specifically) the rabble thereof, viewed as one mob of unruly, disorganized people predisposed to violence and malevolence.
The Mafia: any particular mafia, mentioned uniquely within the discussion's established or implicit context, usually and especially the Sicilian–Italian–Italian-American one (especially in American English) but occasionally others (e.g., the Russian one, the Japanese one).
Any specific one of several historical reigns of terror.
The Reign of Terror during the French Revolution.