Librarian

Space Marine psyker. Less likely to be fucked inside out by daemons though, due to being a superhuman and all. Often overheard punishing xenos and heretics alike while screaming cheesy one-liners like "Your book is overdue, BITCH!".

History

You can tell this dude in front is a Librarian, cause of the HUGE MOTHERFUCKING BOOK on his head.

Back during the Great Crusade, when the Primarchs were reunited with their legions, the Space Marines began discovering some of their new recruits had psychic potential. Wanting to utilize this and prevent them from falling to Warp corruption, Magnus the Red, Sanguinius, and Jaghatai Khan proposed the creation of Librariums, divisions of the Legions which would take in psyker Space Marines and train them to use their powers for the good of the Imperium of Man. Seeing the possible good, the Emperor agreed, authorizing the go ahead and presumably getting all teary-eyed at how independent and grown-up his kids had gotten (while being awesome and manly of course). When the Librariums proved effective, they spread to other Legions. However, a number of Primarchs, namely Mortarion, Rogal Dorn, Corax, and Leman Russ opposed the training of Librarians and wanted the program shut down, believing that the powers of the Warp could not be controlled (Russ maintained that Rune Priests were not actually Librarians, given the Space Wolves insistent maintaining that they drew power from Fenris, not the Warp). Further concern was raised with Magnus's constant peering into the Warp to learn more about his powers, which Russ and Mortarion felt warranted censure.

Then Nikaea Happened

Following the Emperor's departure from the Great Crusade, a conclave was called to determine whether or not Librarians should continue to be used. With representatives summoned to the Council of Nikaea (often remembered as the Trial of Magnus the Red) both sides pleaded their cases. In the end, the Emperor, fearing that Magnus was too close to learning the existence of the Chaos Gods, decided that Librarians posed too great a risk and ordered the Legions to disband their Librariums, return to their battle companies, and never use their psychic powers in battle again. An edict Magnus ignored.

Unfortunately, this was one of those decisions that the Emperor didn't really think through. Without Librarians, only Magnus and the Thousand Sons foresaw the corruption of Horus to Chaos and none of the legions were properly equipped to deal with the daemonic entities and warp-based monstrosities that the Traitors had no problems using.

That didn't stop each Primarch figuring this out quite independently and quietly overturning the edict on their own:

• In one of his brighter ideas, Roboute Guilliman himself stated during the Battle of Calth that the Edict of Nikea needed to be overturned as soon as possible. Thus, when he thought that Terra and the Emperor had already fallen he decided to build the Imperium Secundus in a manner he thought sensible and kept his Chief Librarian around as an advisor (much to the chagrin of a Space Wolves detachment deployed on Macragge to "ensure" Guilliman's loyalty.)
  • (Without a Primarch, the remaining Iron Hands mostly fell in with the Ultramarines and so basically did whatever they were doing.)
• Lion El'Jonson re-introduced the Librarians as soon as he discovered that the traitors were using daemons, presumably knowing what shit was going down from his experiences on Caliban. Even going so far as to kick the living fuck out of a Chaplain who reminded him that it was going against the Emperor's edict.
• Rogal Dorn locked all his Librarians up in the Phalanx and refused to even let them rejoin their battle brothers as line-troopers. However, when Nathaniel Garro stealthed his way on board to try and rescue them, they seemed happy enough where they were. With the Primarch explaining that they were exactly where they needed to be. Presumably for keeping them in reserve for when the Imperium really need them.
• The White Scars were conveniently "away" at Nikaea, so when they returned to the Imperium and found it in civil war, they pleaded ignorance and kept using psykers. It was also suggested that the Khan himself was a psyker and he wasn't going to feel guilty about using his gifts.
• The Space Wolves flatly denied having psykers at all, proving themselves even more ignorant than the White Scars.
• While Sanguinius desperately tried to uphold the Emperor's word, his own former Librarians met in secret and proved their worth by saving the entire Legion from the daemonic precursor of the Black Rage at Signus. So he kept them afterwards.
• There were barely any Salamanders left after Isstvan, though by M41 they looked back upon Nikaea as an ancient and archaic edict that had no place any more.

"Modern" 40k

There is obviously a bit of historical revisionism going on, because in most accounts from the 41st Millenium look back at Nikaea and suggest that the Emperor actually said "yes" to Librarians, so long as they were trained in a controlled and careful manner.

• Which is what the White Scars and Blood Angels were doing at the time anyway, it was only the Thousand Sons who ruined it for everyone by thinking there should be no limits

By M41 it has been left it up to individual Chapters whether or not to use Librarians, and most Chapters had Librarians to some degree or another, with the noticeable exceptions of the Black Templars and the Marines Malevolent.

Ranks

Among Codex compliant Chapters, the ranks of the Librarium are based on their level of training and advancement. They are as follows:

• Chief Librarian: The head of the Librarium, and the most powerful psyker in the Chapter. He is also one of the masters of the Chapter and usually a senior adviser to the Chapter Master.
• Epistolary: The highest rank below Chief Librarian, Epistolaries serve as the main communication aides on the battlefield, sending psychic messages through the Warp to their brothers in the Librariums, similar to Astropaths, but without having their eyes burned out.
• Codicier: Mid-level Librarians, who evaluate reports from campaigns and document them in their Librariums.
• Lexicanum: Entry-level Librarians, they are responsible for compiling battlefield reports for the Codiciers. Interestingly, they don't wear psychic hoods, or even helmets unless in dangerous environments.
• Acolytum: Librarians in training. Brought in from the battle companies before or shortly after they leave the Scout company, they're tested for psychic potential. It's possible that the Acolytum may be killed in the process, but that's life in a Space Marine Chapter for you.