A Black Iron Core is proscribed Imperial technology that was once used exclusively amongst the Battle Titans of the Legio Xestobiax that allow a Titan's crew to use the spirits of the dead to help pilot and animate their god-engine, much like Craftworld Aeldari Ghost Warriors and Titans.
This technology allows a Titan to be effectively piloted with a smaller crew and to respond more rapidly and efficiently to the course of events on the battlefield, though at the cost of sometimes bizarre and unexplained phenomenon.
Developed on the distant and isolationist Forge World of Zhao-Arkhad, the use and development of Black Iron Cores directly violates the Crimson Accord of Mars and has been declared heretek by the wider Adeptus Mechanicus.
During the Great Scouring of the early 31st Millennium, the Metalican Mechanicum Assay Teams officially purged both Zhao-Arkhad and its Legio Xestobiax of this practice. Despite this action, surviving examples of Xestobiax Titans bearing Black Iron Cores would be rediscovered upon Zhao-Arkhad in the late 41st Millennium.
It is unknown if this technology is still used by the Dark Mechanicum or other Renegade factions of the Adeptus Mechanicus.
Located at the very border of the Segmentum Tempestus, at the time of the Great Crusade the Forge World of Zhao-Arkhad had spent millennia in isolation from the wider Mechanicum and other human worlds. Continuously attacked by xenos raiders, the increasingly desperate Arkhadian Tech-adepts considerably deviated from the tenets of the Cult Mechanicus and began to study and use captured xenos technology to guarantee their own survival.
The god-engines of the Iron Vigil benefitted from its parent Forge World's seeming acceptance of the illicit study of various psykana and other related disciplines of half-forgotten and ill-understood lore from the Age of Strife, as well as the strange artefacts scavenged from the hulks of Eldar Titans felled in combat by the Titan Vigils of the Legio Xestobiax.
As such research had never been officially censured within the Arkhadian forge-fanes, even prior to their isolation, the largest of its fanes dedicated its study to this forbidden knowledge while the smallest offered their services through more traditional production capacities.
The arrival of the Thousand Sons Legion on Zhao-Arkhad during the Great Crusade served to kindle the curiosity of these larger forge-fanes in these forbidden technologies and, in particular, the interest of the Arkhadian Eminarii sect under the command of the brilliant Forge-Domini Tacticus Proctor.
Forge-Domini Proctor did not shirk from directly violating the sacred warnings of the Cult Mechanicus. His first success was the creation of a perverted fusion of human and machine, creating hybrid Battle-Automata (robots) whose Cybernetica Cortexes were augmented through synthetically mapped neural networks whose base materials were the cortexes of emergent human psykers.
These Eminarii hybrid Battle-Automata benefitted from almost pre-cognitive levels of response to threat stimuli which considerably strengthened the defences of the Arkhadian forge-fanes. It was only after this success that Magos Proctor would start development of the Black Iron Core.
In essence, a Black Iron Core tries to emulate the Wraithbone cores at the heart of Eldar Titans, which are themselves believed to be far more powerful versions of the simple Spirit Stone carried by most Asuryani warriors. Through intense study of those xenos Titans vanquished and scavenged by the Legio Xestobiax, Magos Proctor developed a psycho-reactive material that largely surpassed the standard Mind Impulse Unit used to pilot Imperial Titans.
The Black Iron Cores seemed to channel the knowledge and experience of fallen Titan crew-members with far more reliable results than the crude foresight displayed by the Eminarii Battle-Automata, allowing the crew to harness a level of skill, co-ordination and tactical prowess that transcended the training of any single Princeps.
This new technology allowed the Legio Xestobiax to operate its Titans with less crew than its counterparts from other Legions of the Legio Titanica. Yet the use of Black Iron Cores and the psychic energy they generated also had its drawbacks. Examinations of fallen Legio Xestobiax god-engines and captured pict-records of their crew would reveal that these altered Titans were prone to a variety of bizarre phenomena.
These included: mass hallucinations of the crew, violent and unexplainable poltergeist activity and even temperature variance within the Titan's hull, especially within those compartments relatively close to the Black Iron Core's operation. In addition, when the Titan's reactors were offline the Black Iron Cores would exert a steady energy drain upon their surroundings, including any biological life forms present.
It has been suggested that the benefits provided to the Legion's Titans by the Black Iron Core technology were partially responsible for the defeat of the lauded Legio Mortis during the Burning of Prospero, yet evidence would suggest that at least in the case of the Prosperine Vigil of the Legio Xestobiax, Magos Proctor -- now promoted to the rank of Forge-Domini of the Arkhadian sub-forge placed on Prospero -- continued to enhance the capacities of the Black Iron Cores available to his Titans.
Rumours have it that with the tacit agreement of the Thousand Sons Legion, Proctor delved into even darker arcana, perhaps even into lore concerning daemonic possession and the creation of Daemon Engines, which would become increasingly frequent amongst the Traitor Titan Legions during the Horus Heresy.
- The Horus Heresy - Book Seven: Inferno (Forge World Series) by Alan Bligh, pg. 180
- Inferno! Vol. 3, "The Spirit of Cogs" by John French, pp. 14-45