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The phrase "power behind the throne" refers to a person or group

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.

*Wikipedia