The existence of a Librarius in the Emperor's Children has been a matter of debate for a few years at this point. This thread is my attempt to gather all references to the Thirds potential utilisation or rejection of such an organisation within the legion.
From Slaves to Darkness:

The Heresy Blackbooks state:
The novel Scorched Earth is possible sighting of a Librarian of the fields of Istvann V.
Reflection Crack'd which originated much of the debate states:
From Slaves to Darkness:

The Librarians of the Emperor's Children have few remaining functions as scribes. Other than making occasional notes of ''amusing'' and ''witty'' deaths they have witnessed. Their main duty is to provide psychic support to the physical struggles of their fellow Legionaries.''
Psykers are particularly highly regarded by the Emperor’s Children, both as enemies and within their own ranks. The broadcast terror of an enemy psyker can be enjoyed in its own right as a new sensation, while a Legionnaire psyker can kill his enemies with pleasure or pure sensation - the greatest act of worship for a servant of Slaanesh. Close combat, where the enemy can be touched and directlyl destroyed, is also much favoured by the Emperor's Children. Few of them enter battle without some form of close combat weapon.
The Heresy Blackbooks state:
''There is also the immutable fact that the Emperor Himself was a psyker of unprecedented power, although the scale of His psychic might was such that He was beyond the perils of the Empyrean, and that He Himself had no doubt deliberately made the Thousand Sons what they were, at least in inception. Furthermore, and with greater discretion, Alpharius of the Alpha Legion, Fulgrim of the Emperor's Children and the Night Haunter- Curze himself was a potent psychic oracle by some accounts- all made active use of psykers within the ranks of their Legions, though perhaps with more reservation and less fanfare. These Primarchs had all supported a proposed scheme of structured training within the Space Marine Legions that would allow psychically talented Legionaries to employ their powers safely and in a directed and regimented manner-an initiative that would become known as the Librarius Project during the mid years of the Great Crusade.''
Should every variation in squad type be found in other legions been present this would include variations of units originating from the Thousand Sons.The Emperor's Children were ordered and precise; formation sizes and structure were largely uniform and where they were not it was either a temporary aberration or a deliberate variation for a particular purpose. Within this careful order the squad was the base unit that showed most variation in both size and function. Comprising a handful of Emperor's Children each squad had a particular purpose and specialisation. Squad members were expected to excel in their allotted roles and would train exhaustively to achieve the pinnacle of efficiency and unit cohesion. Notably all of the variations in squad type and equipment found in other Legions were present within the Emperor's Children, as they believed there was no sphere of warfare they could not or should not excel in. There were however certain beliefs held within the Legion's culrure about the superiority of certain martial virrues over others. Beliefs that originated as the opinions and inclinations of their Primarch Fulgrim and filtered down through the ranks as ironclad doctrine that was not to be questioned.
The novel Scorched Earth is possible sighting of a Librarian of the fields of Istvann V.
Too late, I realised the danger I was in. Lorimarr was a psyker and I a slave to his malicious will.As his fingers touched my battle-helm, just the lightest caress of metal against metal, I was bombarded with a host of painful images. Fire… An endless conflagration and the destruction of a hundred battletanks. A roar of anger, a curse spat from a primarch’s lips in accusation of a brother. Pain and light, so hot it seared the very flesh off my skeleton and turned my bones to ash.I pulled away from Lorimarr’s touch, my ears ringing and a trickle of blood seeping from the corner of my mouth. I wiped it away, about to kill the traitor when I saw that the Emperor’s Children legionary’s eye was open and unblinking. In his last act of attempted murder, he had ended himself.‘Ra’stan…’The voice sounded dim, the edges of my sight still hazed, and odd after images related to my earlier visions assailed me like pieces of a broken kaleidoscope. ‘Ra’stan, are you hurt?’ Usabius was holding me up. Without his intervention I would have fallen, such was the intensity of Lorimarr’s psychic assault. I nodded, my senses returning. ‘He tried to kill you,’ he added, letting me go so I could support myself.‘A Librarian…’‘More like a sorcerer, I think, but yes. ’‘I should not have survived that attack,’ I said, facing my brother. ‘How could I?’‘I don’t know, but you did. Vulkan protects even his wayward sons. ’‘So we might carry on with our mission? ’I did not believe that, but decided not to question the distant providence that had kept me alive. For now, it was enough to know that Lorimarr had been thwarted and he was left for whatever carrion feeders haunted the skies of this place.
Reflection Crack'd which originated much of the debate states:
''They spread out around Fulgrim, each weighing the enormity of what they were doing against the potential for wonder and fresh sensation. That they had managed to subdue a primarch was miracle enough, but to drive out a creature of the warp… Was such a thing even possible Lucius looked from face to face, understanding that no one gathered around the body of Fulgrim could answer that question. The Emperor’s Children had been a Legion reticent in employing Librarians. The genetic quirk that allowed a psyker to wield the power of the warp came about as a result of a genetic mutation, a flaw. And nothing that could be considered a flaw would be permitted within the ranks of Fulgrim’s Legion.''
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