Tarris was a humanoid from a planet that worshiped the Eternal Machine.

At some point, the Third Doctor recruited Tarris for his plan. While the Doctor was to appear in the Sanctuary of Stars to tell the people their religion was a lie, Tarris would protect the Doctor. Tarris concealed a crossbow under his robes and shot a guard during the disturbance caused by the Doctor's announcement. (PROSE: The Discourse of Flies)