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 Menachite Forerunner complex 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.