Butcher's Nails

The Butcher's Nails were bio-neural cybernetic implants, a piece of leftover archaeotech from the Dark Age of Technology. They originated on the world of Nuceria where they were used by 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. The use of these cortical implants were used on the Primarch Angron, when he was enslaved by the ruling elite, 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 their undoing, as Angron gathered his fellow cyber-slaves and instigated a bloody insurrection, reaping a bloody swathe across the settlements and cities of Nuceria. Ultimately, their slave revolt failed, for Angron was transported away by the Emperor of Mankind on the eve of battle and Angron's brother and sister gladiators were slain to a man. Knowing the effectiveness of these cybernetic implants, Angron had his Legion's Techmarines and Apothecaries duplicate 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 archaeotech 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 caste of wealthy elite who lived in decadent opulence while the populace of their cities lived in abject poverty in the slums 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 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 unknown. 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 nor exception, although his Primarch's physiology resisted much they attempted, in one particular method they were successful, and were able to implant deep within his brainstem psycho-surgical devices 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, as first as a novelty expected to die, but soon as a favoured champion who rose swiftly to become the greatest gladiator the world had ever known.

War of Gladiator-King
Little did his so-called "masters" know, or perhaps they did not care, as secure as they were in their overmatching power and their opulent palaces, bitter hatred and resentment 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 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. These gladiator army soon came to be known as the "Eater 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 watched with pride as 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, as the rebellious slaves were butchered to the last man and woman. Angron would never forge the Emperor for this most dishonourable of deeds, foreverafter staining the Primarch's honour, and would continue to fester into a deep soul-wound that would later be used to Warmaster Horus' advantage, to turn his brother against the Emperor during the Horus Heresy.

The World Eaters
Upon taking control of his Legion, the bloody-handed Primarch enacted many changes upon his Legion. Knowing how successful his cybernetic bio-neural implants could be at boosting a warrior's prowess in battle, Angron ordered his Apothecarion to insert the psycho-surgical 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 proved 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 Legion's line units, while those deemed perhaps too unstable for such tasks joined a growing number of near-berserker assault units known as Rampager Squads, and within these 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 to come for the entirety of the XII Legion.

In those earliest days, the Nails were considered a virtue. None of the newly-renamed World Eaters 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 had resisted the chance to share the same virtuous pain as their noble Primarch. Everything changed with the hammering of the Nails. The World Eaters, once known for their brotherhood, became known first and foremost for their savagery. Reports began filtering back of excessive Legion casualties in tacticless displays of horde warfare, and Imperial Army forces pleading for assistance from other Legions when the World Eaters were the ones to answer the call. Planets surrendered rather than face the XII Legion in battle, but not all who surrendered were spared the war. The Nails dulled all other pleasures, until the heady bite of adrenaline was the only certain way to experience anything but the dimmest memory of emotion. Their rewired minds allowed no other pleasure beyond battle.

The rot started to creep within the XII Legion when they had rediscovered their Primarch from his backwater world, yet, the World Eaters could still have refused 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 to be placed inside. Though Angron had ordered the implantation of the Nails, even he couldn't have truly forced a hundred thousand warriors to bend to his will if they'd refused the mutilation of their minds. Deep in their hearts the Legionaries of the XII Legion knew that Angron was broken long before he ever reached them. That was why they had allowed him to have the Nails beaten into their heads. They had hoped that by breaking themselves upon the same anvil, they'd finally fell unity with their gene-father. But it didn't work, and the World Eaters were cursed to eternity of bloody slaughter in the name of the Blood God.

Function
These cortical implants would boost a warrior's adrenaline, resulting in greater strength and aggression in battle. Killing was exhilarating the way nothing else could be, and the Nails played their neurochemical game to make it so. When the Nails sang they stunted the serotonin in the brain to encourage instinctive aggression, just as they deadened emotional response and electrical activity to anything but the flow of adrenaline. Amongst the archeotechnological whims of the Butcher’s Nails, these were the effects the XII Legion had detailed most extensively in their long-abandoned studies. All that cold and considered knowledge meant little. The pain engine in their skulls 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' implants were primitive copies of the malignant original. They erode stability and damaged the subjects’ capacity to reason. They impinged on higher brain function by rewriting emotional responses. However, they were not fatal -- not degenerative in the terminal sense. The most important aspect of their implantation that they shared with the original Nails was that they couldn't be removed without killing the host, or -- at best -- inflicting severe and irreparable brain damage. The Nails weren’t 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 mortality’s instincts. The Nails rewarded rage with spurts of electrochemical pleasure, tingling synapses and deadening enjoyment of everything else. No better machine had ever been contrived.