STO version of the Mirror Universe seems to combine elements of the Shows versions with the IDW Comics version, I just want to make sure neither contradicts the other,
I'll share what I know, Mirror Sato becomes Terran Empress after taking over the USS Defiant and killing Mirror Jonathan Archer,
then we had Clone Sato Empresses until the early 23rd Century where Georgiou then usurped the throne, she then disappeared into the Prime Universe leaving behind a massive power vacuum,
couple of unnamed emperors later and the events of Mirror Mirror happens, Prime Kirk breaks the Prime Directive, since I guess parallel universes are an exception to the rule, Mirror Spock then takes over the ISS Enterprise once Mirror Kirk returns, Mirror Spock becomes Terran Emperor, who then used the tantalus field to get rid of political enemies, made reforms that allowed the Klingon-Cardassian Alliance to conquer the Terran Empire, ends up as a scapegoat and goes into hiding where he has a chance encounter with Mirror Data,
Then we end up in the 24th century where everything gets muddy at this point where the Crew of DS9 crosses over into the Mirror Universe from time to time (Guess Mirror Picard was doing his own thing during the whole Terran Rebellion period), Prime Sisko then hands the Terrans the Defiant Schematics, the Terrans build the ISS Defiant and turn the tide of battle in their favor and rebuilt their Empire,
then stuff happened where we now have God Emperor Crusher.
List of conquered Species, the ones not on this list probably got conquered about the same time their prime counterparts joined the UFP,
https://www.arcgames.com/en/forums/startrekonline/#/discussion/1263644/updated-list-of-terran-empire-species
I edited this thread so it's easier to read.
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-Lord Commander Solar Macharius