Well, the only person above him is the empress herself. Even if he's serving some other master, they would already be a fallen noble since she's unified the entire continent iirc. Unless there's people from elsewhere getting involved, but that would be a bit weird to bring in on this just to make one duke a corrupt twat. They're already so close to the throne at that position, if they're greedy and power hungry they've just got one person and maybe their children to get rid of.There's a good chance that there's not only Duke Fez but others below and above him that have orchestrated this. It's not a one man act. So taking all of this into consideration, I think she made a decent move.