Who is Jagar Tharn?

I have read books like the real Barenziah and he gets a mention in there. was he in an earlier elder scrolls game or something?

If I'm remembering right, I believe he was a doppleganger who posed as the emperor in Battlespire. I've never played the game, so I'm not entirely sure, but I think that's right.

NOTE: I know this thread is two months old (I am aware that I am one month away from thread necro), and I am also aware that it is unlikely that the author of this thread will return to it, but I am giving an answer so that this thread will be useful if someone uses the Search feature.

The UESP answers the "Who is Jagar Tharn?" question well.
http://www.uesp.net/wiki/Lore:Jagar_Tharn

In Morrowind, he is only mentioned in books. This makes sense since he is an important historical figure who was defeated almost thirty years before the beginning of TES3.

List of books he is mentioned in (Source: http://www.uesp.net/wiki/Lore:Jagar_Tharn).
* The Real Barenziah, in particular volumes IX and X. (This is primary reference to Jagar Tharn)
* Brief History of the Empire, Part 4 (some information about his rule over Tamriel)
* A Life of Uriel Septim VII (Tharn as s ruler over Tamriel)
* The Light and the Dark (mentioning Tharn's connection to the Evil)
* Response to Bero's Speech (Tharn as a Battlemage)
* Mystery of Talara, v 4 (Tharn mentioned, with some additional information)
* Words and Philosophy (Tharn mentioned)
* The Doors of Oblivion (Tharn mentioned)
* A Dance in Fire, Chapter 7 (Tharn mentioned)
* Hallgerd's Tale (Tharn mentioned)
* Confessions Of A Thief (Tharn mentioned)