In 1494 DR, during the The March of the Dawn Titan, the Black Network was, in truth, a network divided with the recent power struggle in Waterdeep among Manshoon and other subfactions of the Black Network. During the Titan's march, they were biding their time and waited to see how the matter of the Titan would play out. The Zhentarim, having a rivalry with the Hawthorne Guild as a mercenary company and due to actions members of the guild have taken on multiple occasions, they had no desire to directly assist the adventurers in the fight.
