I have always loved the psyker aspect of GW games. The other worldly space wizard aspect is amazing for creating wonder and horror in a grimdark setting.
Now unfortunately, the VIIth legion did not field its members of the librarius after the edict of Nikea was introduced. They weren't even reintroduced into the rank and file. Instead, Dorn mandated that they should be confined to barracks upon the Phalanx until further notice. Now they weren't imprisoned but they were effectively quarantined. Whether Dorn did this over concerns and mistrust of his gifted sons, or if the primarch could perceive some other purpose for them remains to be fleshed out. Personally i think it was a little of both.
So as i am a super adherent to the fluff, ive not converted a librarian because of this.......but now things have changed. With the timeline creeping closer to the siege of Terra, and a time when all legions used anything they had on hand (including witchy mind bullets) to fight the enemy, i felt a little more at peace with building and using a librarian, mind you only for games set close to the siege of Terra time.
This particular librarian i've converted with the vision of a senior librarian prior to the edict of Nikea being enforced, stripped of position and rank, confined to barracks on the phalanx but then re-instated when everything sideways with Horus's assault on Sol. He has legion command insignia, with Terra unification honours. I modelled him no other weapons other than his force staff to try give the feel his powers are what he has come to rely upon. The pose was intended to not be one indicating direct fighting as i have a preference for the divination powers and i wanted to showing him acting in a support role because of his powers.
And the final touch i wanted to portray was the eagle topped staff and book on his waist, little signs of the Lectitio Divinatus creeping into the legions, almost as a direct opposite to the taint of the warrior lodges. Being a Terran born fist i felt he would be more susceptible to the teachings that the Emperor was indeed a god, even though it being contrary to the Imperial Truth.