The Duke of Northumberland was the father of Guildford Dudley and the uncle of Jane Grey. He convinced Jane's parents to allow his son to marry Jane.
When King Edward died, the Duke put his daughter-in-law on the throne. When Mary tried to take the throne, the Duke tried to rally support. According to the First Doctor, his attempt would be unsuccessful. However, the Duke had been possessed by a Vrij and gathered more followers than history recorded. The Doctor and his companions defeated the Vrij to restore history to its proper course, but they never found out what happened to the Duke. Barbara Wright told the Doctor and Ian Chesterton that Mary had had him executed. (PROSE: The Nine-Day Queen)