10 MORE Unanswered Mysteries From Doctor Who
3. Where And When Is The Valeyard?
The Valeyard presided over the trial of the Sixth Doctor, attempting to spin events to force a guilty verdict. This would grant him the Doctor's remaining regenerations, back when that still meant something in Time Lord society. There was however a twist - the Valeyard himself was said to be one of those future regenerations, purportedly between the Twelfth and final incarnation.
It wasn't as simple as a standard regeneration. He was created, some say, as an amalgamation of all of the worst parts of the Doctor. He was mainly an enemy of his own sixth incarnation, yet returned to engage other versions as well. He would also become instrumental in the Time War.
Much of what is known about him comes from either the expanded universe or from the Big Finish productions but he has been name-checked in the show as well. The Great Intelligence said that he would come again into existence before the end, taking his place in the Doctor's lives. The Testimony also named the Doctor as a Shadow of the Valeyard as well, observed by both the First and Twelfth Doctors.
Where this incarnation remains is hard to say. One story puts him on a time-locked planet, a puppet of the Time Lords, destroying Daleks over and over. Other stories have him still out there, biding his time.