The original followers of Zhentar grew their powers and gradually faded from public view or passed their positions on to others. What the inhabitants of Zhentil Keep did not know was that Zhentar's followers grouped together in the crypts beneath the city and achieved lichdom. Unlike most other liches, the liches, who were later called "Those Who Walk By Night", worked together to expand Zhentar's original goals. However, the passage of time slowly eroded their minds to the point where they eventually rose up to haunt the streets of Zhentil Keep beginning in the Year of the Curse, 882 DR.
Businesses began to close early and people barricaded themselves in inns and taverns. Only the brave or foolish walked the streets at night. It was only after several notable merchants were slain and commerce began to suffer that the Lords decided to take any action. The Zhentilar began to grow as the need for a proper defense mounted. Skilled soldiers, backed by powerful wizards and priests of Bane, swore fealty to the Council of Lords. The wizards in control of the Zhentilar cared little for the threat from the Night Plague, but used it as a convenient excuse to bolster their grip on the city.
They did indeed clash mightily with the liches, but at least a few of those battles were surely staged for dramatic effect. The only outcome from such clashes was the destruction of property of Lords who opposed the Zhentilar. In truth the Zhentilar bribed, coerced, and threatened the liches until they agreed to return underground where they belonged. The only lich truly slain during this time was Vingril, but an adventuring company called the Golden Stag took the credit for that.