Doctor Who: Every Version Of The Doctor Explained
17. The Valeyard (First Appearance: 1986)
The Trial of a Time Lord revealed another mysterious incarnation of the Doctor – the Valeyard.
The exact nature of how the Valeyard came to be was never fully explained in the show, perhaps due to how tumultuous the making of this season was.
Going by what's said onscreen, the Valeyard is a distillation of the Doctor's darkest urges, that originates somewhere between their twelfth and final incarnation. The Master is the one who revealed this information, and they're far from being a reliable source!
The Valeyard did confirm this to the Sixth Doctor however, and warned him that he was inevitable.
It remains to be seen whether or not we'll see the Valeyard again, in one form or another. He could feasibly appear anytime before the end of the Doctor's life, so who knows when he might pop up.