Imagine a spoiled royal, heir to the throne, part of a powerful bloodline that gives them "divine right to rule". They're raised to be cruel, and told their heritage puts them above everyone else.
And then one day, it's revealed before the entire court that it's all a lie. The royal was switched at birth with the child of a servant. The real heir has been serving in the palace all this time, unaware of their birthright. Maybe they're even someone the faux-royal had been particularly cruel to all their lives.
The ruling family is quick to push out the false heir---blood is more important to them than any illusion of family---and welcome the servant with open arms.
Maybe the false heir is banished from the kingdom they were raised to rule. Maybe they're imprisoned so the truth can never come out. Maybe they're made a servant, now at the beck and call of someone they'd thought beneath them.
Does the true heir take pity on them, or do they seek vengeance from years of abuse? Does the royal family have any regrets, or have they always been cold, only concerned with holding power? What do the servants and commoners do, now that the arrogant "heir" has lost all power and protection?