According to the UESP, she left during the Arnesian War, in which Symmachus was killed. However, this is contradicted by her Biography and the UESP's timeline. According to her Biography, she left Morrowind due to rebellion in the city, leaving her husband behind as she travelled to the Imperial City to seek help from the Emperor. However, she found that the Emperor was only Tharn in disguise, and while she was there she was informed that Symmachus had been killed. She then, with Eadwyre's help, befriended Tharn, discovered the location of the pieces of the Staff of Chaos, and promptly left for High Rock. Then Tharn was defeated as per the events of Arena and Barenziah was chilling in High Rock until Daggerfall starts.
So, looking at the timeline, I would assume she was hanging out with Tharn in the year 391, when Symmachus apparently died. Then, five years later, the Arnesian war happens, and she supposedly leaves Morrowind then (wasn't she with Eadwyre?). Then, another three years later, Tharn is finally defeated.
Now then, for my questions:
1. So, was Barenziah in Morrowind or High Rock during the Arnesian war?
2. If she fled Morrowind due to the war, does that mean Eadwyre was hanging out with her in Morrowind until then?
3. And Helseth was with her?
4. If she wasn't, does that mean Llethan was in charge of Morrowind during the war?
Finally, any other knowledge anyone has on the Arnesian war would be very much appreciated. I have had some trouble finding a good source for this, so any books or pages would be helpful too. Thanks a lot everyone, and I apologize if the questions seem silly. I just need to know what characters I can work with in my fan fic.