Librarian

"Fear not the psyker."

– A 10-foot tall psychic super soldier capable of shooting lightning bolts and fireballs with just the power of his mind.

"There are two great errors: First, to pretend that the Path of Heaven does not exist. Second, to follow it."

– Some dead guy from Chogoris, talking about the warp.

I HAVE THE POWER!

Librarians are Space Marine psykers. Unlike Imperial sanctioned psykers, Librarians are less likely to be fucked inside out by daemons, due to a combination of the intense indoctrination regimes of the Space Marines, immense mental fortitude brought by their Librarian training, and the higher quality of their psychic hoods. They are armed with Force Weapons such as a Force Sword, Force Axe or a Force Staff. Often overheard punishing xenos and heretics alike while screaming cheesy one-liners like "Your book is overdue, BITCH!". They have varying degrees of awesomeness depending on what writer and/or chapter. Usually though, they dominate the battlefield and are feared by both friend and foe as they wield a vast selection of pure psykic awesomeness.

History

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. When the Librarium proved effective, they spread to other Legions. However, a number of Primarchs, namely Mortarion (he's got psyker issues that would make a Culexus blush), 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 that the Space Wolves insistently maintained that they drew power from Fenris, not the Warp except not really...). 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

See Council of Nikaea for the highlights.

Long story short, Big Emps banned Librarians, but not everyone listened.

"Modern" 40k

I AM THE SENATE!

There is obviously a bit of historical revisionism going on, because 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 up to individual Chapters whether or not to use Librarians, and most Chapters have Librarians to some degree or another, with the noticeable exceptions of the Black Templars and the Marines Malevolent; the former is for reasons that nobody can explain (some theories include a gene-seed quirk or a byproduct of the Cacodominus's destruction; the Black Templars just shrug and say that it's just the Emperor's will), and the latter is probably because they are the biggest dickheads in the galaxy and the Black Ships make sure they don't get any Psykers for Gene Seed implantations or the fact they could be loyalist Iron Warriors. And now with the creation of Primaris Marines comes their version of the Librarians. Also, seems like Morty compromised in the end.

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. On the tabletop, Chief Librarian is an upgrade, letting you cast and deny one more time and know one more spell. You can even take it if you otherwise have a named Character like Mephiston.
• 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. On the tabletop, this is your basic Librarian, ables to cast two psychic power and deny one per turn.
• Codicier: Mid-level Librarians, who evaluate reports from campaigns and document them in their Librariums. Used to represent the basic ML1 Librarians in older codices, though they've been phased out on tabletop around 8E.
• 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. Haven't been represented on the tabletop since 2nd edition, when they were WS/BS4 W1 A1 Ld8 and a psychic power.
• 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. If they survive they're enrolled in Introduction to Mind Bullets and How Not to Melt Your Head 101.

Gallery

• 
  Standard
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  Standard with Force Axe
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  Terminator
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  Blood Angels Terminator
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  Grey Knights
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  Deathwatch Librarian
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  Primaris Librarian
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  Primaris Vanguard Librarian
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  Varro Tigurius
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  Primaris Varro Tigurius
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  Ezekiel
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  Sevrin Loth

Additional types of Librarian

• Rune Priest - The Space Wolf variant.
• Librarian Dreadnought - Blood Angels Librarians in a box.
• Primaris Librarian - The new kids on the block.
• Sorcerer - The heretic ones.