War Doctor

This Twelfth incarnation of the Doctor was encountered by the Eleventh Doctor and Clara Oswald when they entered the Doctor's time stream on Trenzalore. (TV: The Name of the Doctor)

The Eleventh Doctor, upon coming into contact with this incarnation spurned the very memory of him, going so far as to suggest this incarnation did not even hold the moniker of "the Doctor" due to his actions in life. This unknown incarnation responded by defending those actions, stating he had "no choice" and that he did what he did "In the name of peace and sanity".

This could hint towards The Doctor's actions in the Last Great Time War, and why he is named in the history Clara found in her exploration of the library. (TV: ''Journey to the Centre of the TARDIS).