The Apothecaries of every Chapter test its initiates' physical gene-structure whilst the psychic potential o fthe young candidates is examined by the Chapter's Librarians.
Librarians are powerful psykers. They are responsible for all interstellar communication with the power to project their minds through warp space. They are also adjudicators of the Chapter's psychic population, dividing psychic mutants into those whose powers are dangerous and those whose powers may be used by the Imperium...
Space Marine Librarians are trained to use their powers in battle: to unleash energy blasts, to sense the enemy's movements, and to predict the fortunes of battle. They learn how to read the Emperor's tarot and to sense the subtle movements of daemons and other extra-real entities through warp space. A Librarian can feel the psychic shock waves that herald the arrival of a spacecraft, or the turmoil that ships leave in their wake as they depart. All these powers and more make the Librarians the ultimate Warrior mystics, endowed with abilities which set them aside from their brother Space Marines.
Let's get to the stats:
WS5 W2 A2 and a 3+ save and don't forget about the Force weapon.
Which makes for a very vulnerable HQ character that you need to figure out how to protect.
Librarian. Still a staple. BA powers are still strong, giving you a 5+ cover save on turn 1. Nice to have. Fear of the Darkness has lost some power but if you can double tap a squad they will often run off the board anyway. Note that despite going last, a Librarian with a force axe is AP2 and with S10 up, can put the hurt on.
Honestly, I'm probably going to do another article just focused on the changes to Psykers in this edition, and it's quite a lot to put down in just a paragraph. Basically, I think Librarians came out one of the top winners in sixth due to the sheer number of options you can give them. I know I have been enjoying my Terminator Epistolary (Divination) leading a large group of Bolter Death Company, and I've seen some great things done with a Biker Librarian (Telekinesis).
Overall the field for psykers changed radically, so you really can't just look at them like normal Special Characters anymore. Sure, they have all the same benefits, such as Look Out Sir, Challenges, and all the rest, but they're so much more than that now- now they are a real force to be reckoned with on the battlefield, able to both buff and destroy enemy units at a whim.
Of course, not to neglect our own brand of psychic powers: Blood Lance became an amazing anti-flyer tool, and Unleash Rage is just as good as ever. Fear the Darkness? Well, you can't escort units away anymore, but combine it with some of the new powers and some truly dangerous combinations begin to appear.
In the end psykers are now a huge deal, and I feel they deserve more than just the cursory talk this article will give. We'll speak later over the complex changes and differences psykers make this edition.
-Our libbies got both boosted and nerfed all at the same time. Boosted in that opposing libbies can't stop our powers that help our own units (Shield, Wings, etc), nerfed because our hoods got nerfed too. Mephiston and Libby Dreads are much more reliable now. That said, fudgemuiffin Wolves and Eldar still have their great psy defence, so it's not all roses.
If you are not sure which HQ to take, a Librarian is a good all-around choice. You can give him various weapon loadouts along with a choice of psychic powers. You can't really go wrong for that kind of smaller points investment. You just need to make sure he is protected in a unit or even better in a vehicle.