From Lexicanum

Any Space Marine that shows any psychic potential is separated from the rest of his battle brothers and trained by the Chapter's existing Librarians. In time the trainee may show sufficient control over his powers and join the Librarium. Librarians are powerful psykers; able to rip apart space or slow down time, and are a force to be reckoned with. Equipped with a Force Weapon and psychic hood, a Librarian is adept at disrupting any attempts by enemy psykers to use their powers, and is formidable in close combat. Additionally, Librarians are also responsible for keeping the Chapter's history, and counselling the Chapter Master on how the Chapter is best served in the future.

The Chapter is continually vigilant to ensure that recruits will not taint the gene-seed of the Chapter through mutation. However, the psyker mutation is often encouraged, and these Marines are inducted into the corps of Librarians. These psychically-charged warriors are trained to use their abilities in the field of battle, as both weapons and tools of precognition. Off the field of battle, the Librarians are responsible for maintaining and adding to the records of the Chapter, attending to the Chapter's annals and the collective knowledge built up over millennia of service. The Librarians use a psychic hood to suppress their enemies' use of psychic powers; the hoods' wires tap into the brain and act as an extension of the wearer's consciousness and allow him to tell when a being taps into the warp. It detects the manipulation of the warp and focuses the Librarian's will to disrupt it through a series of augmentic crystals.

The path to becoming a Librarian is a difficult one, as they must not only be strong enough to survive their training, but possess enough mental discipline to fend off the daemons and entities of the Warp, as these creatures see the enhanced form and mind of a Librarian as a strong prize. Also, the Librarians are often isolated from the rest of the Chapter, as they carry with them the abilities the Space Marines are expected to abhor and purge in all other beings.

Contents 1 History 2 Chapters 3 Ranks and Roles 4 Sources

History

Originally there were no Libarians in the Legions. Preheresy, the first were introduced, but there was a great dispute among the Primarchs if this would be right, at least Psykers are a large danger, the mightier the more dangerous. So it needed a decision from the Emperor. He allowed the introduction into the legions, but they should wear blue armour so every brother could spot that the brother astartes before him is a psyker and might be in danger of possesion (or worse). During the Great Crusade, Librarians began to adopt an advisory role for the certain chapters they were affiliated with, providing a voice of reason to commanders on the ground during the conflict. Suprisingly though, the Librarians were psykers and thus considered untrustworthy and dangerous. It was often proved that Librarians were the most staunch supporters of the Emperor within their chapters. In fact, a number of Traitor Legions were first forced to kill their entire Librarian divisions in vicious battles before they could successfully turn against the Emperor.^{[Needs Citation]}

Chapters

Several Chapters have different names for Librarians such as the Space Wolves who call them Rune Priests. Some exclude their use entirely, like the Black Templars. Other chapters, such as the Blood Ravens, revere Librarians greatly, having a higher likeliness of them being included in their forces.

Ranks and Roles

There are four battlefield ranks of Librarians, which describe both their position and role.¹

1. Lexicanian: Prepares reports of battles for their chapter's records.
2. Codicier: Evaluates those reports and provides strategic overview of campaigns.
3. Epistolary: Chief psychic communications officer, receiving and transmitting psychic messages on the battlefield.
4. Chief Librarian: Reports directly to the chapter's commander. Holds overall responsibility for the maintainance of communications lines. Scrutinises battle reports, giving recommendations for honour awards.

Sources

• ¹Genestealer (1990, Space Hulk expansion)

Space Marine Infantry
Command Units	Master • Captain • Master of Sanctity • Reclusiarch • Epistolary • Codicier Command Squad • Apothecary • Company Standard Bearer • Company Champion Terminator Command Squad • Ultramarines Honour Guard
Flexible Troops	Tactical Squad • Terminator Squad • Veteran Squad • Scout Squad • Tyrannic War Veterans
Combat Troops	Assault Squad • Terminator Assault Squad
Ranged Troops	Devastator Squad
Support Units	Space Marine Bike • Space Marine Attack Bike • Scout Bike