Crypt of the Shadowking — Mark Anthony

When the evil Zhentarim attempt to take over the richest of the Caravan Cities, an ex-Harper agent tries to foil the wicked plot
 
Under a dark cloud, Caledan Caldorien left the Harpers seven years ago. Now a mysterious and beautiful messenger from the secret society convinces him to undertake one final, reckless mission.
 
Iriaebor, the City of a Thousand Spires, has fallen under the sway of the evil network of Zhentarim. With a fellowship of old friends, Caledan must seek an ancient magic to counter the dark conspiracy of the Harpers' nemeses. For awaiting them all is a terrible secret that lies beneath the city . . .
 
Crypt of the Shadowking is the sixth book in a series of loosely-connected novels about the Harpers.