First of all, wow, that's a lot of younglings for a dark elf.

The Nightingales reveals that Barenziah first conceived with Drayven Indoril, and the result of this coupling was Dralsi, Karliah's mother. Her second child was Tiber Septim's, who was terminated before birth. Third and fourth were Helseth and Morgiah, of course. But The Real Barenziah mentions in its final volume that Barenziah had a fifth child, a son by Jagar Tharn, who was born and grew up in the court at Wayrest. Is there any information on this one beyond that which comes from the books? I don't believe he ever appeared in any of the games and it seems like a bizarre oversight.

I'm pretty sure the one who was born to Jagar Tharn was Drayvens. Remember, according to Gallus Barenziah claimed they were the same when in actual fact Jagar employed Nightingale.

But several factors suggest that these were different children. First of all, Barenziah's child with Nightingale was a girl, and TRB says that Tharn's child was a boy. Second, she got rid of Dralsi because she was inconvenient to her story, but she officially recognized her son and raised him at Wayrest (or at least stated her intention to). And the timing is off, since (at least to me) Dralsi seemed to have been conceived when Indoril seduced Barenziah at Mournhold, but she was still pregnant with the mystery son during the events of Arena.

EDIT: Not to mention, The Nightingales mentions that Barenziah was "fond" of Jagar Tharn, so it isn't inconceivable that she could have been with both. You know what they say about dark elf females...