Spymaster

"O" reveals himself as the "spymaster". (TV: Spyfall)

While the Thirteenth Doctor was searching for Kasaavin-affiliated spies, MI6 agent "O" openly pondered as to who was their spymaster. Infact, "O" himself was not only in league with the Kasaavin spies, but actually a new incarnation of the Master. Revealing that his reference to a spymaster was a clue for the Doctor, the Master identified himself as the "spy... master." (TV: Spyfall)