Librarian

Space Marine psyker. Less likely to be fucked inside out by daemons though, due to being a superhuman and all.

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 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 Librariums proved effective, they spread to other Legions. However, a number of Primarchs, namely Mortarion, Rogal Dorn, Corax, and Leman Russ. 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 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. In the aftermath of the Horus Heresy, and now realizing that Librarians were a first line defense against possible Chaos corruption, the High Lords of Terra chose to cease enforcement of the Edict of Nikaea, and left it up to the individual Chapters on whether or not to use Librarians.

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.