Off of the top of my head:

- The first two Trails of Cold Steel games set inside the fantasy Prussia expy. Technically there is an imperial civil war and you fight against the faction that advocates for foreign expansionism, but the protagonists still fights against the enemy superpower that is encroaching upon their country of Erebonia. The sequels sadly have the protagonist commit treason and aid foreign countries against his own.
- Vanguard Bandits: early on there is a route split and you can swap from fighting with the rebels against the empire, to joining the imperial princess and her knights and fight within the empire against the internal usurper faction.
- Tyranny: whole gimmick is that it is The Black Company but as a video game but the protagonist is 100% loyal to the dark lord.
- Two Fire Emblem games, Fates and Three Houses, have a route where you fight for the cool evil empire.
- Some MMOs have a playable imperial faction (Amarr empire in EVE Online, Dominion in Wildstar, Sith Empire in SWTOR, etc).






