Thomas Percy was one of the plotters in the Gunpowder Plot of November 1605. Though he was responsible for placing the Scottish King James VI on the English throne, he turned against him when James reneged on his promises to treat Catholics with tolerance. He subsequently joined Robert Catesby's plot to kill the King.
On the night of the plot, he was guarding the door to a meeting between Robert Catesby, Guy Fawkes and Lady Winters. Amy Pond frightened him by baiting rats, interrupting them, and allowing Rory Williams to steal a document that showed the Plotters' next meeting point once they all left. (GAME: The Gunpowder Plot)