The Zhentarim are indeed about to attack Loudwater. They have allied with a group of devils and a surviving member of the Calishite Rensha family who used to rule over Loudwater. Otar, the Green Regent is leading an expedition to Silverymoon to see if he can gather allies to help defend against the upcoming attack.
Part 3 of the Tides of War miniseries was never published, somewhat spoiling its effect on the campaign. Information on the plot was taken from the following module.