From Warhammer 40k - Lexicanum
The Hexarchion Vaults are a part of the Librarius Omnis rediscovered by Arkhan Land during his first expedition. They contained profane texts that Land deemed a moral threat and potential perversion of cognition, including information on the cerebral implants known as the cruciamen. The vaults were resealed by Land's decree, ratified by Fabricator General Kelbor-Hal, with all findings unrecorded.
During the Horus Heresy, Land used forbidden knowledge gained in the vaults to aid in the ascension of Magos Domina Hieronyma into the Archimandrite at the behest of Fabricator General Kane.
- 1: Master of Mankind (Novel), Chapter Seven