KING UNDER THE MOUNTAIN is the nomenclature used to refer to any files pertaining to the planet Onyx. It was assigned on March 6, 2525 by Vice Admiral Margaret Parangosky. It was also the name of a file on Colonel James Ackerson's computer at CASTLE Base on Reach, containing files related to Onyx, and other material closely related to the Forerunners as well. The other file on Ackerson's computer was called S-III.
The KING UNDER THE MOUNTAIN file contained:
- Information on the Forerunner artifact found on Sigma Octanus IV.
- The debriefing of John-117 after the Battle of Sigma Octanus IV.
- A full map of the CASTLE base facility on Reach that included a path to the immense Forerunner facility as well as the Forerunner Crystal, beneath the Menachite Mountain.
- A map of the complete survey of Menachite Mountain, where the original mineral mines were located before they were taken over by ONI. Menachite Mountain was an extinct volcano which is where Dr. Halsey found the Forerunner Crystal artifact.
Due to the order that all materials and files relating to Onyx was to be redesignated under KING UNDER THE MOUNTAIN, it is possible that Ackerson was using the Forerunner data in the file to find technology to use against the Covenant or for his own personal means.
- The name may refer to two different uses of the phrase. The Dwarven king of Erebor, from J.R.R. Tolkien's universe, was titled King Under The Mountain. Another real-life use of the phrase, or a similar one, is King In The Mountain, a common European folklore belief that a legendary or mythical hero lies asleep beneath a mountain awaiting the day they shall reawaken and return to lead their people, notable King Arthur and Merlin of British Arthurian legend, Emperor Charlemagne of France, Emperor Frederick Barbarossa of Germany, and the Aztec hero-god Montezuma. In this second interpretation, the crystal would symbolise a relic intentionally left there to be recovered eons later for an important use.