Skies Over Cormyr
The Zhentarim are either the worst spymasters in Faerun, the wiliest traders, or some combination between the two.
Once focused on military conquest, the Zhentarim refocused their efforts on economic methods after the Empire of Netheril disappeared. As Cormyr kept its attention focused on Sembia the agents of the Black Network once again began infiltrating the city-states around the Moon Sea region. Locals unhappy with the rule of Netheril tossed their lot in with the Zhents, preparing to stand and fight should the Shadovar ever come back, and the Zhentarim haven’t let go since.
Zhent business practices are designed to maximize Zhentarim gains at the cost of whatever reputation they might have. Their diplomats are top-notch, their lawyers the best money can buy, and rarely are Zhent law-breakers kept within the confines of jail for too long.
Every city in Faerun has a Zhent caravan exiting or entering its gates daily, and the Black Network has grown far across Faerun. The growth of the rule of law hasn’t hurt them.
Of course, everyone knows someone who knows someone who had an arm or leg broken because they initially refused to do business with Zhents, but few will say so before an officer of the law, or in court. Zhent thugs are..unused to opposition, and the way to the top in the organization requires equal combinations of ruthlessness and craftiness.
There is a history of Bane worship in the ranks of the Zhentarim, and shortly before the conquest of the region by Netheril the ranking Banite cleric in the world was the head of the Zhentarim organization.
Centuries later, any Zhentarim encountered will swear up and down that Bane-worship amongst the Zhentarim is an old stereotype, perpetuated by those with an axe to grind against the Zhents. Due to Zhent business practices, however, finding someone with an axe to grind against the Zhents is not difficult.
If there is one weakness amongst the Zhents, it’s due to the way the organization works. While few know exactly what occurs in Zhent high councils, there is no definitive chain of command. A few reports speak of the same tactics being used for internal disagreements as are used for external matters.
(IRL description: Think of the Italian city-states run by the Cosa Nostra. Business organizations tied together by common interests, forbidden from outright warfare upon one another, but competetive nonetheless. Here, though, the Cosa Nostra is out in the open, maintains a sizeable military, and has had 300 years to build a trade network across the world. One thing remains constant, however, faced with an external threat the Zhentarim stand together…as long as its advantageous for them to do so.)