War Doctor

An unknown incarnation of the Doctor was encountered by the Eleventh Doctor and Clara Oswald when they entered the Doctor's own timestream. It was implied that this incarnation of the Time Lord was responsible for a transgression that dishonoured his name.

The Eleventh Doctor, upon coming into contact with this incarnation, spurned the very memory of him. He went so far as to suggest this incarnation did not even hold the moniker of "the Doctor" due to his actions in life. He explained to Clara that his real name was not the point; he chose the name "the Doctor" and explained that the name a person chose was like a promise. This unknown incarnation was "the one who broke the promise." He described him as "my secret." This unknown incarnation responded by defending those actions, stating that what he did, he did "without choice" and "in the name of peace and sanity." The Eleventh Doctor acknowledged that as true, but rebutted that it had not been "in the name of the Doctor." As he carried Clara away, the unknown incarnation turned around and watched them with a sombre stare. (TV: The Name of the Doctor)

Behind the scenes

If this Doctor is an actual incarnation of the Doctor (and not a copy or a manifestation of some part of the Doctor's personality), then Hurt, aged 73 when he first appeared on screen, will become the oldest actor to portray the Doctor upon their debut appearance. The second oldest would be William Hartnell, who was 55 years old when the show premiered in 1963.
Strax describes this incarnation of the Doctor as having "the look of a battle hardened warrior." (WC: Strax Field Report: The Doctor's Greatest Secret)