The Zhentarim are a shadowy group centered in the Moonsea area of the realms, north of the Sea of Fallen Stars. They worship primarily the god Bane and are dedicated to the accumulation of power across the Realms. However, they are more likely to try and extend their influence through control of trade routes and infiltration of existing political structures than direct conquest.
Disaster at Dun Helm:
The Zhentarim have over the course of the past two years successfully infiltrated the local government of Tannersvale by bribing Councillor Avador. They orchestrated a hobgoblin attack so as to install a mercenary group in the town to start taking over tradesmen’s duties. Now that they have been repelled, it is unclear what repercussions there will be, though it has been intimated that Tannersvale will be feel the wrath of the Zhentarim and their allies.
The Zhents seem to have taken their defeat at Tannersvale in stride. Chauvlin intimated that this was a minor setback which will be rectified in time, but you get the sense that the reach of the Zhentarim is long and widespread. So it is only a matter of time before Tannersvale is targeted again. Based upon a ledger you found in the Unstrung Harp, you believe that the Zhemtarim may have been trading goods through the town for a very long time.
The reason it has become a target now seems to be a combination of a lack of organized resistance and the importance of the river now that overland trade routes controlled by the Zhents have become perilous.