Eugene Tacitus was a prisoner in a pocket universe that looked like a castle. He had murdered his son; insanity caused him to commit the crime. In the castle, he was the author of the inhabitants' actions. He acted as scribe and writer of religious text. After a new god was elected, their bible had to be rewritten. Over time he grew unaware of his role as author. He constantly repeated the actions of building a world before it was destroyed by his son, whom he then murdered.
Eugene recorded all major events. He often wrote that events had been predestined, to calm the public. After the death of Pepin VI, Sejanus had Eugene recant the belief that he was a god. Pepin VII was to be the new god, and Eugene was to believe it under penalty of death.
Eugene spoke to Pepin VII before his coronation, instructing him in his actions. He questioned Eugene as to his belief in the process of "god-making". Eugene recorded Pepin's coronation, trying to make him sound brave. Pepin declared that he was not a god.
The Sixth Doctor and Frobisher arrived and were treated as godly ambassadors. They announced they weren't, but Eugene saved them and Pepin by saying previous gods had experienced a period of mental fatigue and Pepin was merely confused.
With the Doctor
Eugene was present during the assassination initiation of Pepin. Eugene took the Doctor to the library, actually a collection of bibles in Eugene's bedroom. They were taken prisoner by Childeric, who showed them the catacombs where he had imprisoned his son, whom he hoped would rule in the future.
Eugene and the Doctor were imprisoned. Childeric wanted Eugene to continue his work as a scribe, recording the actions of his son. However, this would require his tongue to be cut out, as Childeric did not want his son to be tainted by hearing speech. They escaped while Childeric was distracted by his son. Eugene broke the circle by having the apparition of his son kill him. (AUDIO: The Holy Terror)