Ratatoskr was the leader of the 47th Marauding Horde of Ongatatis 5. He resembled a very skinny rodent with two large tusks extruding from his mouth and had yellow eyes, who wore partial armour on his head and his shoulders. He also had his own champion, Rataberserkr who he's use to challenge champions of different planets to take over. He was familiar with the Shadow Proclamation, but not entirely fulfilled on the rules, after buying into a lie by Nardole about Article 2367 of the Shadow Proclamation, despite there only being 2366.
At one point he arrived at St Luke's University to challenge the Doctor to a test of endurance against his champion. However he was gone, and only Nardole was there to challenge him. After dressing up at the Doctor he invoked Article 2367 of the Shadow Proclamation to choose the challenge. Nardole used Harry as his champion for a challenge of climbing up the statue and putting a traffic cone on its head and balancing it, for he knew that this was a prank he played often. Harry won, and Ratatoskr was annoyed that he was tricked, but due to their honour as warriors they left. (PROSE: I Am the Doctor)