The Butcher's Nails, referred to in profane texts in the Hexarchion Vaults as the Cruciamen, were cybernetic cortical implants, a form of archeotech that dated to the Dark Age of Technology and were used extensively by the Space Marines of the World Eaters Legion, precipitating their fall to Chaos.
The implant technology originated on the world of Nuceria where they were used by the ruling aristocracy on gladiatorial slaves to enhance their fighting abilities, turning their already formidable fighting stock into ravaging killers, barely controlled between each gladiatorial bout.
These cortical implants were also used on the Primarch Angron when he was enslaved by the ruling Nucerian elite of the city-state of Desh'ea, turning him into a killer the likes of which they had never before seen, Angron Thal'kyr -- Lord of the Red Sands, the greatest gladiator the world had ever known. This would prove to be the Nucerians' undoing, as Angron gathered his fellow slaves and instigated a violent insurrection, reaping a bloody swathe across the settlements and city-states of Nuceria.
Ultimately, their slave revolt failed, for Angron was teleported away by the Emperor of Mankind on the eve of battle and Angron's brother and sister gladiators were slain to the last warrior. Knowing the effectiveness of these cybernetic implants in enhancing combat effectiveness, Angron eventually had his XIIth Legion's Techmarines and Apothecaries reverse-engineer the Butcher's Nails, using his own implants as a template. The resulting implantation of the Nails within the Legionaries of the World Eaters turned them into a Legion of savage killers and would lead to their eventual downfall and damnation as they reveled in the bloody worship of the Blood God Khorne.
Lord of the Red Sands
The archeotech devices known as the Butcher's Nails originated on the world of Nuceria, located on the outskirts of the Segmentum Ultima. A technologically advanced world, Nuceria was ruled over by a wealthy elite who lived in decadent opulence while the populace of their cities lived in abject poverty in the slums that surrounded their walled palaces and villas. To distract the populace from their poverty, the oligarchic rulers of Nuceria held regular gladiatorial deathmatches in massive arenas, using cybernetically-enhanced gladiators who battled to death to satisfy the endless bloodlust of the oppressed people. It was on this world that the Primarch Angron was eventually discovered, though little else about the circumstances of how he came to be there remains known. It was a matter of course that the slave masters used biochemical and cyber-surgical enhancements as well as relentless and brutal training to "improve" their fighting stock. Angron was no exception, although his Primarch physiology resisted much of what the slavemasters attempted. But in one thing they were successful, for they were able to implant deep within his brainstem psycho-surgical devices intended to augment his aggression to inhuman levels and turn him into a superhuman beast, a killer the likes of which had never been seen. The young Primarch was bound by the slavers and gladiatorial masters, first as a novelty expected to die, but soon as a favoured champion who rose swiftly to become the greatest gladiator the world of Nuceria had ever known.
War of the Gladiator-King
Little did his so-called "masters" know, or perhaps they did not care, secure as they were in their unmatched power and opulent palaces, of the bitter hatred and resentment that burned within Angron's heart for his captors. His only comradeship was with the gladiatorial warriors with whom he fought. Long had he plotted rebellion and vengeance, and soon he shattered his bonds and led a bloody revolt, staged amid the largest Death Games the arena of the city of Desh'ea had seen in generations. With his gladiator kin rallying to him, they reaped a bloody swathe across the planet as city after city fell before their wrath. This gladiator army soon came to be known as the "Eaters of Cities," but despite snatching victory time and again against fearful odds and the superior ranged weaponry of the planet's elite, Angron knew his revolt was ultimately doomed to failure.
Worn down by attrition and hunger, Angron and his brothers and sisters were determined to stand and die in honourable battle rather than being taken alive. Far above in orbit, the Emperor of Mankind had watched with pride how Angron had led his outmatched revolt, but now chose to intervene, unwilling to accept His lost son's death in battle no matter how resolved and honourable the cause. Upon confronting Angron and making an offer of a place by His side, to His surprise He was refused and rejected. The Emperor would not accept this and on the eve before the final onslaught forcibly teleported the enraged Primarch away from the slaughter that followed at the Battle of Deshe'lika Ridge, as the rebellious slaves were butchered to the last man and woman. Angron would never forgive the Emperor for this most dishonourable of deeds, forever after staining the Primarch's honour. It was a stain of guilt and shame that would continue to fester into a deep soul-wound that would later be used to the Warmaster Horus' advantage, when he sought to turn his brother against the Emperor during the Horus Heresy.
Upon taking control of the XIIth Legion, the bloody-handed Primarch enacted many changes. Knowing how successful his cortical implants could be at boosting a warrior's prowess in battle, Angron ordered his Apothecarion to insert the Butcher's Nails implants within every Astartes of the World Eaters Legion to enhance aggression and pain tolerance far beyond that which even the gene-engineered flesh of a member of the Space Marine Legions was capable. This posed difficulties, however, as Angron's implants were relics of a long-lost technology, little understood even by their makers, while removing them from Angron for close study would have proven fatal to the Primarch. The Legion's Techmarines and Apothecaries attempted to duplicate the process using the Primarch's own implants as templates to reverse-engineer the devices.
Early attempts to duplicate these implants were far from successful, and resulted in high rates of mortality and irrecoverable homicidal frenzy on test recruits. However, as time progressed, a viable form of the cortical implant technology was replicated and steadily improved, although it was never fully stable or constant between subjects, and entire newly-formed companies of recruits were implanted, as well as large numbers of existing World Eaters who volunteered for the dangerous operation. The majority of these Astartes were absorbed back into the XIIth Legion's line units, while those deemed perhaps too unstable for such tasks joined a growing number of near-berzerker assault units known as Rampager Squads, and even within these units, those too far gone to be anything but restrained as savages between battles became known as the Caedere or the "Butchers," a frightening portent of what was soon to come for the entirety of the XIIth Legion.
In those earliest days, the Butcher's Nails were considered a virtue. None of the Astartes of the newly-renamed World Eaters Legion would face the fact their Primarch carried a curse from the years on his homeworld. They focused on his prowess, on the strength and speed gifted to him by the archeotech implants, and when the Primarch demanded his sons lie under the Techmarines’ claws and the Apothecaries’ knives, few resisted the chance to share the same virtuous pain as their noble Primarch.
Everything changed with the hammering of the Butcher's Nails into the skulls of the Astartes of the XIIth Legion. The World Eaters, once known for their brotherhood, became known first and foremost across the nascent Imperium for their savagery. Reports began filtering back of excessive Legion casualties in tactic-less displays of horde warfare, and Imperial Army forces pleading for assistance from other Legions when the World Eaters were the ones to answer their call for Astartes support. Entire planets surrendered rather than face the XIIth Legion in battle, but not all who surrendered were spared the war. The Butcher's Nails dulled all other pleasures, until the heady bite of adrenaline unleashed through the stress of combat was the only certain way for a recipient of the implants to experience anything but the dimmest memory of emotion. Their rewired minds allowed no other pleasure beyond battle and slaughter.
The rot had started to corrode the XIIth Legion as soon as they had rescued their Primarch from his backwater world, yet, the World Eaters could still have refused to receive the Nails. They chose to emulate their gene-father, despite all it would cost. They chose to mutilate themselves and to tear open their skulls and let the poison of the Nails be placed inside. Though Angron had ordered the implantation of the Nails, even he could not have truly forced a hundred thousand warriors to bend to his will if they had refused the mutilation of their minds. Deep in their hearts, the Legionaries of the XIIth Legion knew that Angron had been broken long before he ever reached them. That was why they had allowed him to have the Butcher's Nails beaten into their heads. They had hoped that by breaking themselves upon the same anvil, they might finally feel unity, a sense of kinship, with their gene-father. But this weakness only bred corruption, and the World Eaters eventually found themselves cursed to spend an eternity in bloody slaughter in the name of the Blood God.
In the aftermath of the betrayal of the Istvaan III Atrocity, the World Eaters Legion, under their savage Primarch Angron, became ever more insular as a Legion and uncontrollable on the battlefield, proving a double-edged sword even to their allies. The psycho-surgery rife within the Legion became even more widespread and extreme in its use, and the Neophytes inducted with ever-increasing pace into the World Eaters' ranks to replace the fallen were cerebrally mutilated with the Butcher's Nails implants as a matter of course. All-out infantry assaults supported by fast moving armour, with the aim of immediately closing into bloody melee with the foe, had always been a hallmark of the Legion, and now became often their goal; carnage for its own sake beyond any strategic objective to the contrary.
The cortical implants known as the Butcher's Nails would boost a warrior's adrenaline, resulting in greater strength and aggression in battle. Under the perverse influence of the implants, killing proved exhilarating in a way nothing else could be, and the Nails played their neurochemical game to make it so. When the Nails activated, they stunted the production of the neurotransmitter serotonin in the human brain to encourage instinctive aggression, just as they deadened all other forms of emotional response and neuroelectrical activity to all parts of the brain save for that which regulated the flow of adrenaline. Any attempt to remove the implants would prove fatal, as the brain became entirely dependent upon this regulation of its neurotransmitters in only a short time. Amongst the archeotechnological whims of the Butcher’s Nails, these were the effects the XIIth Legion had detailed most extensively in their long-abandoned studies. Of course, that cold and considered knowledge meant little to those afflicted with the implants. The pain engine implanted in the skulls of the World Eaters forced them to enjoy killing above everything else. Even the comfort of brotherhood paled -- one of life’s few remaining pleasures outside of battle.
The World Eaters' version of the Butcher's Nails cortical implants were but primitive copies of the malignant original lodged in the brain of their Primarch. They eroded mental stability and damaged the subjects’ capacity to reason. They impinged on higher brain function by rewriting emotional responses. However, they were not fatal as the original implant was to Angron -- they were not degenerative in the terminal sense. The most important aspect of the implantation that the Astartes of the World Eaters shared with the original Butcher's Nails was that they could not be removed without killing the host, or -- at best -- inflicting severe and irreparable brain damage. The Nails were not implants as Remembrancers and archeotechnicians understood the idea. The implants added nothing to a World Eater’s brain. Instead, they stole from it. They bleached a warrior’s mind of all reason, all caution, all the instincts of mortality. The Nails rewarded rage with spurts of electrochemical pleasure, tingling synapses and deadening enjoyment of everything else. No better device for the creation of berserk, psychopathic killers had ever been created by the human mind.
But the depths of the flaws inherent to the use of the implants became truly obvious when the World Eaters Librarians gifted with the Butcher's Nails lost the ability to control their psychic talents in battle. One of them, a warrior attached to the 100th Company, had been lost to the madness created by the devices in his very first battle after implantation, and immolated three squads of the 100th Company when he could not cease projecting witch-lightning from his eyes. Several other implanted World Eaters Librarians had just ... burst. They had combusted in pyres of flaming gore. More and more died -- none right away, but they never survived for long. In a single month, almost every World Eaters Librarian had been fitted with the Butcher's Nails. Mere weeks later, they all started dying.
Optimism, albeit cautious, had reigned for a while. After the first deaths, the psychically trained Legionaries had sought to master the implants, to balance their sixth sense with the bionics now altering their brains’ biochemistry. It was all just a matter of willpower they had said, and their Battle-Brothers had pretended not to notice the desperation in their eyes. It was all a matter of willpower -- yet the Librarians kept dying. The Librarians died in battle, in storms of fire or lightning, or -- in several incidents -- by pulsing hateful pain directed through the implants of nearby World Eaters warriors that forced their non-psychic kindred to suffer cerebrovascular blockages. Entire squads died of brain haemorrhages and strokes at their Codiciers’ boots.
Angron ultimately gave his psychic sons a choice between execution and the removal of their Butcher's Nails implants. The XIIth Legion learned, in those early years after their Primarch’s rediscovery, that they had mutilated themselves in the image of a man without mercy. The Butcher's Nails could not be safely removed; every World Eater knew it, for the Emperor’s own techno-mages had failed to remove the Primarch’s implants after his recovery from Nuceria. Even so, most Librarians submitted themselves for the attempt. Every one of them died, without exception. With their rewired brains misfiring and enslaved to altered impulses, none of them died easily, and none of them died well. Soon enough, the last Librarians of the XIIth Legion were those who had not yet received the Butcher's Nails in a Legion now defined by the implants. The Librarians of the XIIth Legion came to eke out an isolated existence in the near-empty halls of their Librarius aboard the World Eaters' flagship Conqueror.
One by one these Librarians, too, began to die. Not from malfunction or misuse, but because they were World Eaters, and all World Eaters lived brief, violent lives. A hundred remained. Then fifty. Then twenty. No one mourned them. In a Legion that prized the bonds of front-line brotherhood above all else, the silent psychic Battle-Brothers died alone -- never forgotten, but always ignored. Their gene-seed rotted with their bodies, unharvested in case their genetic legacy resulted in the same psychic curse infecting a second generation. Vorias, the eldest of the remaining Librarian coven and Lectio Primus of the XIIth Legion's Librarius Division had worked with the World Eaters Apothecarion and their senior attached Mechanicum Archagos in trying to determine just why the Butcher's Nails reacted so poorly in the presence of psychic minds, but the line of research was abandoned when they had come to realise the context of their work: no one cared. No one but those cursed with a sixth sense. Besides, their efforts had always ended in vain, and killed too many "loyal" World Eaters who were unfortunate enough to be near the unstable Librarians.
The very presence of psykers caused their non-psychic Battle-Brothers pain, and the XIIth Legion had no need of their psychic gifts. The XIIth Legion had moved on and left their psychic brothers behind to rot away within their ever-diminishing coven. By the beginning of the Horus Heresy in the early 31st Millennium, there were only 19 Librarians left within the entirety of the XIIth Legion. They considered themselves War Hounds rather than World Eaters on account of their lack of the Butcher’s Nails. But their fate was sealed upon the homeworld of Angron's youth, where they were brutally murdered by their own Primarch after his apotheosis to become a Daemon Prince, removing their psychic taint from the World Eaters forever and sealing their bloody pact with the Blood God as his loyal servants.
- Index Astartes III, "Chosen of Khorne - The World Eaters Space Marine Legion," by Graham McNeill, pp. 36-41
- The Horus Heresy - Book One: Betrayal, pp. 86, 88-89
- The Horus Heresy - Book Two: Massacre, pp. 215-216
- Butcher's Nails (Audio Book) by Aaron Dembski-Bowden
- Betrayer (Novel) by Aaron Dembski-Bowden
- The Master of Mankind (Novel) by Aaron Dembski-Bowden