During the Time of Chaos before the Gallifreyans became Time Lords, Arcalian Provost Tepesh, Lady Ouida and Arata comprised the Committee of Three: an investigative group authorised by the Gallifreyan Inner Council to look into the work of Cardinal Rassilon. With the aid of the Great Mother of the Sisterhood of Karn and the High Priestess Cassandra, Tepesh broke into Rassilon's Foundry.
Inside, he found Rassilon, and quickly discovered evidence of the Cardinal's work to create the ability to regenerate and his efforts to lock all timelines into one cohesive Web of Time. As part of this, Rassilon had found and trapped a force he called the Divergence in a pocket universe, fearing their power to destroy the Web of Time. As an added measure, Rassilon had also made sure that the humanoid form was the only dominant life form in the universe via a series of biogenic mutagens scattered on tens of thousands of worlds.
Tepesh, in a fit of fury at Rassilon's ego, revealed that the Committee were in actuality Great Vampires, and that Tepesh was a direct descendant of the King Vampire. Shortly after revealing the Vault where he had imprisoned the Divergence, Rassilon activated a failsafe that firestormed the interior of the Foundry, incinerating Tepesh and the others. (AUDIO: Zagreus)
While dealing with the aftermath of the explosion of anti-time, the TARDIS created a series of holographic projections, taken partly from reality and partly from the memories of the Doctor, Charley Pollard and the TARDIS itself. One such projection was the event at the Foundry, looking for information on the Divergence. Inside this projection, it placed Charley in the role of Rassilon and itself in the role of Tepesh's cohort Arata. The projection of Tepesh was created using the image of the Sixth Doctor.
Soon after, the TARDIS was drawn into a special corner of the Matrix and reduced to scrap by Rassilon and Zagreus. Before being destroyed, though, it recreated projections of Tepesh, Reverend Matthew Townsend and Walton "Uncle Winkie" Winkle in the Matrix to help return fragments of the Doctor's mind to him. The projection of Tepesh had a rematch of sorts with Rassilon, but was killed with a dagger forged from the essence of the TARDIS.
Provost Tepesh was seen in various projections, both in the TARDIS and in the Matrix. These projections contained facets of the Doctor, so Tepesh's appearance was strongly reminiscent of the Doctor's sixth incarnation, enough for Lady President Romana to initially mistake him for the Doctor. (AUDIO: Zagreus)