Here is my counts as Mephiston. Chief Librarian Lucius Thurinus

Base model is the Librarian on foot with bolt pistol. I modified the sword arm into the plasma pistol. The sword arm is a bit that was garnered from Abaddon.I filed down the edge to bring it line with the "modern" fine line weapons. As opposed to the "old school" blunt edge. The hand and arm have been replaced so that the pose is a little more dramatic.

The paint is still a mess and requires a good amount of finish work to get it to the level that I want. Still haven't decided on the toga. I was going to paint it blue but the model would look to much like Superman for the army. 

Now to the reason I chose to do the Counts As instead of painting the actual Mephiston model. The Mephiston model is so recognizable that he did not fit with the army. I wanted something unique for the army. 
As always let me know what ya think

Yes he is carting around a "daemon" sword. 
The story is that he was leading an ill fated recruitment mission. Upon landing, he was seized by visions of Sanguineous walking this world. It was during a search for possible candidates that a small cult to Tzench had infected the world. Since the mission was one of recruitment only the captain of the 10th, the company's sanguinary priest, and a few veterans of the first company were present. It was during the fight the cultists somehow managed to open a warp-rift. Lucius and the veterans was dragged through the portal. The Captain was sure that they were lost. He then sacrificed himself to close the rift as he could see that demons were gathering to come through. As he slaughtered the first horror to come through, he called for the lance strike that would end his life and seal the rift. Centuries lated the Grey Knights, responding to an open warp rift, came upon a small battle weary band fighting at the mouth of the rift. It seems that what was sealed was a rip  in the web way. The Knights descended upon the demons with zeal and after the battle was over and the rip sealed once more. The band was taken into custody, to determine if they were pure and untouched by Choas. 
Lucius and brothers had fallen to the Black Dream, as it is known to the Storm Acolytes. They had fallen and survived. During his interrogation, Lucius reviled that a group of multicolored dancing Eldar had happened upon them, and he had saved their leader, known as Solitaire, from a Keeper of secrets by using the Eldar's sword. During a lull in the fighting, the Solitaire gave Lucius his sword as a gesture of gratitude. A hideous ugly thing, that seemed to have a life of its own. When he received the sword as his own, the sword revealed that is was a construct, dedicated to the destruction of demons, especial those of the Great Devourer. After months, if not a year, the Knights declared that all were clean of the taint of Chaos, and they were taken to the fortress monastery of the Acolytes. After further testing, by the chapter, they were restored with full honors. Yet Lucius had grown far beyond any of the other Librians, his visions were now of a predictive nature, and they have saved the chapter from disaster on many occasion. It seems that without fail, when Lucius has taken the field, the being know as the Sanguinor has shown up and bestowed his blessing upon a Sargent or veteran for that battle. Yet alas the blessing comes with a price, as the received is either grievously wounded or killed in action.