War Doctor

This version of the Doctor appeared at the end of TV: The Name of the Doctor portrayed by actor John Hurt. It is currently unknown which incarnation of the Doctor he is.

This regeneration of the renegade Time Lord was initially seen within the post-death timeline 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 they were "In the name of peace and sanity".

Regardless of incarnation, the inclusion of this unknown regeneration into the canon of the Doctor Who mythology would at least make the known regenerations of the Time Lord one further regeneration along. Christopher Eccleston's portrayal of the Ninth Doctor now technically being a portrayal of the Tenth Doctor and so forth.

Speculation as to the specific generation of this time lord suggests that he is actually the true Ninth Doctor and the actions being referenced were the events leading up to the end of the Last Great Time War.