I don't have copy of Lord of Souls, but in the Imperial Library there's a quote "Mazgar’s mother was killed at the sack of Orsinium, when Mazgar was only seven." which suggests a way to estimate the date. Is there an indication of her age in 4E 43, when the events are happening? Or better yet, an actual date for the event?

http://www.uesp.net/wiki/Lore:Orsinium#The_Siege_and_Destruction_of_Orsinium

No date to be found here.

Really?

You're off by three eras.

Ah yeah. This line then, and there's nothing more precise: "Orsinium was once again sacked by the combined forces of Hammerfell and High Rock in the early http://www.uesp.net/wiki/Lore:Fourth_Era,"

I'd put the sack around 4E10-15, following the unrest in the Imperial City and the succession of Alinor. It'd make Mazgar 35-40 in Lord of Souls, which sounds right to me.

That's about the range I was thinking, too. Which would mean that relations between Orsinium and High Rock have to deteriorate fairly rapidly at the start of the fourth era. At the time of Oblivion, Orsinium and Wayrest are allies, and 10-15 years later it's sack the city. Unless "High Rock" in the context of the sack was "except for Wayrest".

High Rock has always been at odds with Orsinium (which is part of the reason why people are up in arms over the Daggerfall Covenant). The only reason Orsinium got recognition again in the 3rd Era is because they got a big stompy robot that could kick the Time God in the nuts, and the Empire told everyone to play nice. It'd make sense that once the Empire collapsed, they'd quickly degenerate back to fighting.

The Alliance between Wayrest and Orsinium was Elysana's doing. Something must have happened to her during or following the Oblivion crisis.

Well Elysana's not the most trustworthy type, so she could have just double-crossed Orsinium when she saw which way the wind was blowing. Old scores and a weak Imperium's probably enough to let it happen.

The reason I'm asking is that I'm adding quests to one of my mods for the period after the Oblivion MQ - i.e early 4th era, so I'm looking for external events to connect with. It looks like events leading to this conflict would be in the right timescale.

Presumably at about the same time as the Mage's Guild splits into the College of Whispers and the Synod too! Could be interesting.