The Valeyard is an amalgamation (read: mash-up of personalities) somewhere between the Doctor's twelfth and final incarnations that is made up of the darker and evil parts of his personality. The Valeyard was first revealed to viewers during the Sixth Doctor's reign when he wished to steal the Doctor's remaining regenerations to live a life of his own and be free from his "other influence". Peter Capaldi's Doctor definitely fits the criteria for the Valeyard's appearance. Note that a regeneration was not mentioned in there, as the Valeyard could come to being by way of a personality break, where his hatred and vitriol for whoever killed the Time Lords could become a separate entity from the Doctor, causing the "Real Doctor" to try to stop him at all costs. Stranger things have happened during the course of Doctor Who, of course, but if they want to end Series 9 on a happy note...