What if the Red Tooth Rite worked when used by one Noble on another? If you kill a Power and eat his heart, his Domain becomes one of your secondary Domains and you grow in might. Consume enough Domains, and you become an Imperator, tearing free from your old superior to become his equal. Now, you just have to watch your underlings to make sure they don't eat too many hearts themselves, for if you lose all your Domains, you will fade into nothingness.
It's a viciously dark variant of Nobilis, where the gods themselves are at war with each other, trusting no one. It's like Vampire with open diablerie and god-like power. You probably don't even need Excrucians—your potential friends are already your own worst enemies. If I were to run this, I'd probably set it in a future age after the Excrucians have been defeated. Alternately, Excrucians might still pose a very real threat, or at least that's what the Imperators claim. Are they telling the truth, or are Excrucians just a lie invented to direct the Powers' efforts elsewhere and help keep them in line? Are mimics really Excrucians, or just Imperators playing charades to keep up the lie?