Wiktionary
n. (context idiomatic English) Someone who appears to be without special status, but who has great covert influence on a person in authority.
Wikipedia
The phrase "power behind the throne" refers to a person or group that informally exercises the real power of a high-ranking office, such as a head of state. In politics, it most commonly refers to a spouse, aide, or nominal subordinate of a political leader (often called a " figurehead") who serves as de facto leader, setting policy through possessing great influence and/or skillful manipulation.
The original concept of a power behind the throne was a Medieval-era figure of speech referring to the fact that the king's policies could be set by a counselor not seated in the throne but standing behind it—perhaps whispering in the king's ear—out of common sight. Among the earliest examples of such powerful advisors were the Mayors of the Palace under the Merovingian kings.
Usage examples of "power behind the throne".
The wicked, idolatrous queen soon became the power behind the throne.
Wouldn't do to let him know that Rod knew he was the power behind the throne.
At present he was willing to be her ally, but she knew that he would like far better to be the only power behind the throne.
Already he has caused it to become current gossip that he is the real power behind the throne of Lutha--that your majesty is but a figure-head, the puppet of Von der Tann.
No Nithathil aided another without an ulterior motive: did Tanarchal see himself as the power behind the throne?
Right now he could enforce a claim to being the power behind the throne and make it stick at gunpoint, but he couldn't sit on the sacred seat himself.
Someone would have to serve as figurehead while the PCC remained the power behind the throne.
Ysa would, of course, continue as the real power behind the throne.
The great Demerzel, the power behind the throne, the man who has twitched the strings attached to Cleon I all these years and those attached to Cleon’.
The great Demerzel, the power behind the throne, the man who has twitched the strings attached to Cleon I all these years and those attached to Cleons father before him?