The X Doctor

The X Doctor is described as a "personification" created when The Doctor entered into his own to timestream to save Clara. In his own words, he is The Doctor's entire history manifested as living being. Consequently, he takes the form of an as yet unseen future Doctor.

Creation and encounter with The Eleventh Doctor
When The Doctor entered the remnants of his own timeline in order to retrieve Clara Oswald, it is believed the dormant regenerative energies in his body were enough to revitalise the timeline "healing the scar tissue" and causing the collected timeline to congeal into a single being.

The X Doctor was born with an inherent desire to protect The Doctor's history as part of an in-built survival instict, appointing himself "curator" of The Doctor's history. Though initally he only intended to be a guardian and an overseer, the meeting of the Tenth, Eleventh and War Doctors, and subsequent tear in reality, forced him to intervene.

The X Doctor pulled The Eleventh Doctor out of his respective point in time with the intent of recruiting him to help repair the anomaly, as he only existed as an astral projection with limited control over time. However, it become aparant that the Valeyard was using the tear as a conduit to rebuiled himself, having stored a small portion of himself in The Keeper of The Matrix.

The two Doctors were pulled into a twisted alternate timeline, but were able to find and defeat the Valeyard, with a little help from an alternate version of [http://tardis.wikia.com/wiki/Sixth_Doctor The Sixth Doctor. ]

After closing the tear permenantly, The X Doctor attempted to warn the Eleventh Doctor about the dangers of altering one's own timeline, a warning which The Doctor did not heed, instead leaving, still intent on changing his fate for the better.

The X Doctor then went into stasis, unsure of weather he would be needed again.