Shoondale! Have the Leader be a descendant of Manshoon the Great!
"These things also I have observed: that knowledge of our world is to be nurtured like a precious flower, for it is the most precious thing we have. Wherefore guard the word written and heed words unwritten and set them down ere they fade . . . Learn then, well, the arts of reading, writing, and listening true, and they will lead you to the greatest art of all: understanding." Alaundo of Candlekeep
So I'm guessing the Zhents could dig up a heir from the populace...using their detect DNA spells?
I'd expect it would be more like finding a distinctive item known to have belonged to a specific person -- it would likely be an heirloom passed down within a family. Once you find it, you can check back thru local histories and such to find the connection.
Yeah that sounds like a good adventure hook. I can imagine the PCs getting involved in a struggle to locate this heir. Maybe he or she has fled Teshar with a harper agent (possibly wounded in the process), with Zhentarim in pursuit. Maybe he/she is privy to some secrets Teshar don't want the Dales to know. I can imagine a chase adventure with the PCs racing to meet with the Dales Council for him or her to speak.