Butcher's Nails - Flaws: The World Eaters' version of the Butcher's Nails implants were in fact 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 would prove 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 true cybernetic implants as Remembrancers and archeotechnicians understood the concept. 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 solar month, almost every World Eaters Librarian had been fitted with the Butcher's Nails. Mere solar 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 own codiciers' boots.Angron ultimately gave his psychic sons a choice between execution or 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 in their Legion 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 archmagos 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 in the Legion 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 with the implants 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, Nuceria, during the Shadow Crusade where they were brutally murdered by their own primarch after his apotheosis to become a Daemon Prince of Khorne, removing their psychic taint from the World Eaters forever and sealing their bloody pact with the Blood God as his loyal servants.