em ‧ i ‧ nence grise /ˌemɪnɒns ˈɡriːz $ -nɑːns-/ BrE AmE noun ( plural eminences grises ) [countable]
[ Date: 1800-1900 ; Language: French ; Origin: éminence grise 'gray eminence' , originally used of Père Joseph, adviser of the French minister Cardinal Richelieu ]
someone who has unofficial power, often secretly, through someone else:
Borusa had no wish to become president, instead preferring to operate as an eminence grise behind the scenes.