1
general::
A person with great influence who stays behind the scenes and has no apparent authority. For example, Harry may be the CEO, but it's obvious that his wife is the power behind the throne. [Mid-1800s]
American Heritage Idioms
2
general::
Phrase(s): the power behind the throne
Fig. the person who actually controls the person who is apparently in charge. • Mr. Smith appears to run the shop, but his brother is the power behind the throne. • They say that the vice president is the power behind the throne.
McGrawhill's American Idioms And Phrasal Verbs