"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.
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. 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.
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 (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.
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 it 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 now with the creation of Primaris Marines comes their version of the Librarians. Also, seems like Morty compromised in the end.
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 they are usually represented by named special characters of psychic mastery level 3 (or level 2 if they occasionally have some other unique ability). If you don't have a special character in your chosen chapter, then you'll just have to settle for a cool Epistolary.
- 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. While no longer explicitly stated in the 6th edition codex, Epistolaries have traditionally been psychic mastery 2 on the tabletop with the ability to know and cast two powers per turn.
- Codicier: Mid-level Librarians, who evaluate reports from campaigns and document them in their Librariums. Supposedly represented on the tabletop as the common variety "Librarian", only at level 1.
- 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.
Additional types of Librarian
|Forces of the Blood Angels|
|Command:|| Apothecary - Brother-Captain - Brother-Sergeant - Chaplain |
Chapter Master - Librarian - Sanguinary Priest - Techmarine
|Troops:|| Assault Squad - Devastator Squad - Sanguinary Guard - Scout Squad |
Tactical Squad - Terminator Squad - Veteran Squad
|Death Company:||Death Company - Death Company Chaplain - Death Company Terminator|
|Walkers:|| Contemptor-Furioso Dreadnought - Death Company Dreadnought |
Dreadnought - Furioso Dreadnought - Librarian Dreadnought
|Transports:||Drop Pod - Razorback - Rhino|
|Vehicles:|| Baal Predator - Land Raider (Land Raider Crusader - Land Raider Phobos |
Land Raider Redeemer - Land Raider Angel Infernus)
Predator Tank - Vindicator - Hunter - Stalker - Whirlwind
|Flyers:||Stormhawk - Stormraven - Stormtalon - Storm Eagle - Thunderhawk|
|Allies:||Space Marines - Primaris Marines|
|Forces of the Dark Angels|
|Command:|| Apothecary - Company Master - Interrogator-Chaplain |
Chaplain - Librarian - Techmarine - Deathwing Knight
Dark Angels Grand Master
|Troops:|| Assault Squad - Deathwing - Company Veterans |
Devastator Squad - Scout Squad - Tactical Squad
Ravenwing Black Knight - Watcher in the Dark
|Walkers:|| Dreadnought (Venerable Dreadnought - Contemptor Dreadnought) |
Contemptor-Mortis Dreadnought - Mortis Dreadnought
|Transports:|| Land Raider (Land Raider Phobos - Land Raider Crusader |
Land Raider Redeemer - Land Raider Ares
Land Raider Solemnus Aggressor) - Rhino - Drop Pod
|Vehicles:|| Bike Squad - Razorback - Predator - Vindicator - Hunter |
Stalker - Whirlwind - Land Speeder Vengeance
|Flyers:|| Dark Talon - Nephilim Jetfighter - Stormraven - Storm Eagle |
|Allies:||Space Marines - Primaris Marines|
|Forces of the Grey Knights|
|Command:|| Brotherhood Champion - Grand Master - Librarian |
Techmarine - Chaplain - Apothecary
|Troops:|| Grey Knight Paladin - Interceptor Squad - Justicar |
Purgation Squad - Purifier - Strike Squad - Terminator Squad
|Walkers:||Dreadknight - Doomglaive Dreadnought - Venerable Dreadnought|
|Vehicles:|| Land Raider (Land Raider Crusader - Land Raider Vortimer) |
Rhino - Razorback
|Flyers:||Stormraven - Stormtalon - Stormhawk - Thunderhawk|
|Allies:||Inquisition - Sisters of Battle|