Sector 45 - Crimson Banner

Crimson Banner is a star sector colonized quite late, and in fact not fully 'synchronised' in terms of ethnicity and societal origins with the rest of the Eastern Orion Cluster. It was colonized by various refugees from the Terran Federation, during the period of the Reconciliarist movement.
Relative weakness of the new colonies allowed sufficiently ambitious and amoral person to do thing that is considered impossible in the current Space. Admiral Randall Griffith of the Terran Federation Navy went renegade, and assembled a vast host of deserters, outcasts, mercenaries and various revolutionaries.. and led assault upon the Sector 45, quickly conquering it whole and establishing the Stellar Empire.
While strong - and brutal practically on the level of Fourth Reich's Nazis - it suffered heavily from the fact, that unlike the ascendant evil empires of the Unification Wars, it couldn't just midsculpt citizens into compliance. It was a brutal totalitarian regime, with Griffith posing himself as an Emperor (and even going so far as to think about destroying or permanently blocking hyperlanes to isolate his Stellar Empire to no longer have to worry about outside threat)... but it was opposed.
It took several decades of brutal oppression for a revolution to finally occur... and avoid getting squashed instantly. The alliance of several major revolutionary groups succeeded in toppling the Stellar Empire during ten years long total war. The war ended when Zachary Neumayer, the leader of the revolution, struck a deal with a newly ascended Emperor (whose father died in the space battle against admiral Madeline Pike, a hero of the revolution, and right hand of Neumayer).
The new emperor (Alexander Griffith), was 12 years old and, frankly, probably didn't know a lot about what was hapenning. The deal was struck between the Revolution and moderate elements of the former regime - the effect was the creation of the vestigial Stellar Kingdom, and the eventual fall of the rest of the sector by the People's Coalition for Equality and Freedom. Remaining one of the few united sectors.