The Black Network was formed over a century ago to advance the goals of the evil wizard Manshoon. The organization was deeply infiltrated by the churches of Bane and Cyric, who eventually began tearing it apart with sectarian conflict. They became enemies of Netheril, who destroyed their two most important headquarters in the Moonsea region. By the time of the Spellplague, the Zhentarim was already all but destroyed.
In the century since then, the Zhentarim has re-emerged as a widespread mercenary company. This was the public face of the Network before, and some worry that it may be a front now, as well. They are known for hiring out top-quality mercenaries and not shrinking away from illegal or unethical business. Bregan D’aerthe is their most direct competitor in the region.