Cymril was the senior Harper agent in Neverwinter and the founder of the Sons of Alagondar. Her death under mysterious circumstances led to a rift between the Harpers in Neverwinter and the Sons, each blaming the other for Cymril’s death.
Her Harper protégé, Marcus de Tylmarande, and Branwen Farlong, one of the founding members of the Sons of Alagondar, eventually discovered that Cymril was, in fact, a Banite Zhent. Her true mission was to help Dagult Neverember secure an empire on the Sword Coast. Such an empire could check the expansion of Netheril and would ultimately lay the groundwork for establishing a Banite emperor over the entire region. She created the Sons of Alagondar in order to draw out those who opposed Neverember. Most of the Sons naïvely accepted the high casualty rate of the missions Cymril led as the price of their revolution, without suspecting that she was delivering them to Neverember’s mercenaries.
The senior leadership of the Zhentarim are all vampires, due to Orbakh’s influence. Cymril rose high enough so that when she died, she rose again as a vampire. She confronted Branwen and Marcus, ostensibly to offer them the opportunity to join her. Branwen refused flatly, but Marcus said that he would consider the offer.