... No, there haven't. What we did with the Grand Highblood wouldn't work on her, and it's hard to say what would when she is what she is. She hasn't been Empress just for a few centuries; she's been around for longer than I even know. She is a wickedly talented conqueror who's taken over countless planets. She's squashed rebellions, kept us all stuck in line with these rules of society, and has never been budged from her throne. Heiresses are supposed to go and challenge her at a certain age, a battle to the death, and obviously she's never lost. I don't even know how we'd start planning.