Ace in the hole
Something that can supply a sure victory when revealed.
This originates from the game of poker, where a card dealt face down and kept hidden is called a 'hole card', the most propitious card being the ace.
The earliest printed citation I can find is from the Iowa newspaper The New Era, June 1886:
"Thus matters went on until four cards lay in front of each man, face up and one turned down. Not a pair in sight and everyone thought each man had an ace 'in the hole'."
The term was used as the title of a 1951 film, starring Kirk Douglas. The plot involved Douglas's character delaying a rescue in order to cash in on the notoriety of having exclusive reporting rights to the rescue attempt.