That's what Newsweek magazine called the Department of Justice Office of Legal Counsel. Why?
According to Newsweek magazine, the OLC:
is the most important government office you've never heard of. Among its bosses -- before they went on the Supreme Court -- were William Rehnquist and Antonin Scalia. Within the executive branch, including the Pentagon and CIA, the OLC acts as a kind of mini Supreme Court. Its carefully worded opinions are regarded as binding precedent -- final say on what the president and all his agencies can and cannot legally do.
("Palace Revolt" by Klaidman, Taylor Jr. and Thomas. Newsweek, February 6, 2006)
Why is it so powerful? Well the historical commission of the office was to advise the President on what was legal to do. This would apply to all executive orders and proclamations the President made. For example, if the President wanted to make a treaty with another country for such and such, the Office of Legal Counsel would advise him on the legality of it. If it was illegal, then the President would not do it. Sounds nice, right?
Well in the Bush Presidency, the role has slightly changed, that according to former head of the office Jack Goldsmith. In his new memoir The Terror Presidency,
Goldsmith claims this role was nuanced slightly. Instead of advising on the legality of the President's potential actions, the office started using their memos to justify the President's actions by finding legal precedence or extrapolating that legality through existing (but sometimes extraneous) legal documents. For instance a definition of severe pain used in defining unacceptable torture practices was extracted from a medical insurance legal document citing death and organ failure. This way, if the President did anything that turned out illegal, he could say that he was only acting on advise from his lawyers, who "must have gotten it wrong" when they said it was legal. See how this works? Deniable culpability which gives the President Carte Blanche to do whatever he wants....