Background
of Khelben "Blackstaff" Arunsun
By Steven Schend to the Forgotten Realms Mailing List
"Lady Cassandra Thann is the daughter of Zelphar Arunsun.....
Zelphar comes to Waterdeep with young daughter in tow; eventually meets and marries Lhestyn {whose maiden name I don't recall}.
Lhestyn & Zelphar have a child named Khelben; this child will grow up and be called Khelben the Blackstaff untill a distant ancestor shows up and takes the kid's identity. The kid then takes a hike off into the planes and eventually ends up on Greyhawk Oerth as Khelben Ravencloak.......
Despite the secret switch (known only to a select few in the Realms and the readers), most people honestly believe he's the son of Zelphar & Lhestyn (though we know he's about 800+ years older than that). Thus, Cassandra still treats him like a little brother, and any times he seems older than that she mentally chalks it up to "his mysteries and wizards' training." While Dan is astute and probably figures that his "uncle" is far more than that, he's playing along with this charade as it's how the Blackstaff wants to play it. Of all the folks on the Realms, I'd guess that only Elminster, Khelben himself, the Seven Sisters (or at least those he helped raise), and a few select elves in Silverymoon, Evermeet, and Cormanthyr might know his secret; and, of course, they've got good reasons not to tell anyone too....... Even the other Lords are not in on this secret, though Mirt & Durnan suspect, while Piergeiron merely trusts Khelben on faith.... "
Steven Schend
Khelben's Conscience