Father Octavian was a Bishop Second Class who had deployed Doctor River Song on a covert mission. This led to her presence on the Byzantium, which housed a Weeping Angel. He had great respect for the Doctor, knowing of his many feats. Octavian took his job seriously, putting up with River as best he could. In his last moments, he made sure that the Doctor did not learn more than he had to about why River was in prison.
Learning of a dormant Weeping Angel, Octavian had River Song released into his custody to help in return for earning her pardon for the "murder" of the Eleventh Doctor. Octavian had her infiltrate a party being held on the Byzantium to confirm the Angel's existence and try neutralising it.
When he was called down to the crash site of the Byzantium, Octavian was displeased with River, until she introduced the Eleventh Doctor, the "equivalent of an army" she had promised earlier. After learning they would need to infiltrate the Maze of the Dead below the Byzantium to reach the Angel, Octavian had the clerics set up explosives.
While discussing their plan, Octavian asked Song whether she had informed the Doctor of her true identity. Octavian made sure she kept her mouth shut about it, lest he refuse to help.
He led his men into the maze with the Doctor, Amy and River. Three were soon killed by the Angel from the crash. The Doctor assured him they could not have been saved, but Octavian reminded him that he would be the one to tell their families while the Doctor went off in his little blue box. Nonetheless he still told the Doctor he had faith in him when they were surrounded by Angels. (TV: The Time of Angels)
After entering the forest vault, he claimed the he and River were engaged so he could accompany the Doctor and her to the flight deck. On the way he revealed she was in his custody. At the flight deck, when River had already gone through, Octavian was attacked by an Angel. It was frozen, trapping Octavian in a headlock while the Doctor looked at it. Octavian told the Doctor that River had killed a man who was "a hero to many", but he didn't want to know who it was. He warned the Doctor not to trust her. The Doctor refused to leave him, but Octavian told him to go, glad that his courage had not deserted him in the end. When the Doctor tearfully said he wished he had known him better, Octavian replied that the Doctor knew him at his best. His neck was then broken by the Angel as the Doctor left. (TV: Flesh and Stone)