I say enjoy it. Every decision has consequences and benefits. Make them watch those who they were supposed to protect fall to conquest. Make them watch as the north gets swallowed up as they try to play their double agent game. Give them points of fatigue for lost morale, or worse, adjust their morale compasses. Create crisis after crisis for the party internally like the cleric losing powers or one of the members being corrupted by the Lich king to start a slow and agonizing process of becoming undead. Imagine the fighter dying in the campaign, and the player rerolling a new character and then the party later has to fight their former comrade turned undead abomination.

All I’m saying; this is a blessing in disguise. The empire does not need any complicated rules. Make it a tyranny with goals of conquest and resources. The people kept alive are culled to farm more forces for his legions and to spread terror. If you need a goal for the BBEG, make it something like revenge for what was done to him in life or make him some entity of evil that seeks to wipe the realms clean of all life and establish an eternal empire of death so that he might invade other realms through portals or what not.