Rise of Tiamat
The Zhentarim—also known as the Black Network—is a loose affiliation of merchants, mercenaries, and malefactors. Common folk know the Zhentarim as the people to talk to when you need the best guards or mercenaries money can buy. And if the cargo is shady or the cause you’re fighting for is questionable or even unjust, the Zhentarim don’t mind. Affiliation with the Zhentarim can be profitable to sellswords, thieves, and rogues of every stripe. It’s plausible that adventurers in your campaign might become members of the organization—but like most residents of Faerun, they won’t know the full extent of the Zhentarim’s operations and goals.