Thornhold

When a distant family secret threatens the Harpers and the city of Waterdeep, it falls to one rogue agent to save them all
 
A new Zhentarim threat lurks in the shadows of Waterdeep and all is not well in the City of Splendors. The roguish Harper agent Bronwyn Caradoon—a part-time adventurer with knowledge of ancient texts and lore and a penchant for counterfeit jewels—is challenged with a quest that becomes deeply personal.
 
For the good of the city, Bronwyn is sent by Archmage Khelben Arunsun on a mission to meet her long-lost father and reclaim her bloodline's dangerous heritage. She uncovers a family secret that threatens to destroy not only Bronwyn, but the Harpers themselves—and thus learns of the deep connection between her people and all of the races in the realms.

Thornhold is the fourth book in the Song & Sword series and the sixteenth book in the Harpers series.