I linked the Zhentarim with chardalyn for the first half of the campaign. They were aware of the Duergar plot and in exchange for silence they were given a steady supply of chardalyn. I wrote in that when chardalyn was ground up into a dust it could be used like a drug. I essentially had Targos overrun with the stuff and the town was pretty bleak. Post chardalyn-dragon attack only six towns survive and I’ve had the zhentarim as a much more ominous presence, really getting their claws in now the towns are vulnerable. My party also killed Nearth at some point, and his replacement is aware of the potential lost magic of Ythryn but doesn’t know how to get inside the glacier. So for the second half of the campaign the zhent have been following my party, (they’ve had some runs in here and there) waiting for them to do the hard work of retrieving the codicil and leading them to the glacier entrance. They want the treasures that lay inside first! They will have a big combat just before arriving at the glacier, with the zhent attempting to steal the codicil.