The Librarian has three special abilities. Some of his abilities also require research so some are unavailable in early campaign missions.
Smite: Delivers a powerful psychic blast that damages most units and can disrupt infantry. Small area of effect. Best used on clumps of ranged infantry.
Weaken Resolve: Demoralizes a target squad. Requires several moments to be effective. Use it whenever it's charged.
Word of the Emperor: Nearby infantry units are protected by an incredible psychic shield and thus refuse to die as long as this lasts. They still receive damage, however they just stop at 1 health point. Arguably the librarians most useful ability.
The Librarian is a secondary commander that functions similarly to a "psyker" (i.e., has psychic abilities) of the Imperial Guard. Although the Librarian is tough enough to handle several Infantry squads on its own, its main purpose is to summon abilities that support the surrounding Infantry and Heavy Infantry squads that are engaging the enemy on the front line. The support Librarian provides a higher squad morale bonus than a Sergeant, though not as high as the Force Commander.