Wiktionary
n. (alternative form of éminence grise English)
Wikipedia
Grey Eminence: A Study in Religion and Politics is a book by Aldous Huxley published in 1941. It is a biography of François Leclerc du Tremblay, the French monk who served as advisor to Cardinal de Richelieu. He was also known as Father Joseph and as l' éminence grise; that phrase originally referred to du Tremblay.
Political and economic commentator David P. Goldman called this book "[t]he best book on the intelligence operations of the French state" during the Thirty Years' War.
Usage examples of "grey eminence".
But she is, perhaps, the foremost link between her father, the Great Orator of the Industrial Radical Party, and Charles Babbage, the Party's grey eminence and foremost social theorist.
BUCHAN was on terms with Ernie Delgado too, the grey eminence of the Canal.
All of our time will go to some grey eminence who hasn't had an idea in her head for fifty years.