In ‘Halo: Uprising’ by Brian Michael Bendis, a captured soldier saves the Earth using the key of Osanalian, which is something non-existent, concocted out of fantasy games with his brother when they were kids, that only the duo knew of – as a key to a magical secret. The first brother tells his alien captors that they would need the key to defeat humans, and that it was situated where the second brother lives.
When this was announced, the latter catches the hint that his brother was trying to save him, by preventing the city he was at from being destroyed as the aliens would need to search it for the key. The latter seizes the opportunity to say he is the key per se, and surrenders to the aliens. He straps bombs with him and wipes out the aliens, thus turning the war around. The secret key of no key was what saved the day – a fascinating example of how fantasy can be of real use with skilful means! Then again, the true key was the combination of compassion and wisdom.