Wikipedia
Valeyard
The Valeyard is a fictional character from the long-running British science fiction television series, Doctor Who. He is described by the Master as an amalgamation of the Doctor's darker sides from between his twelfth and final incarnations. In the serial The Trial of a Time Lord, comprising the whole of Season 23 of the series, the High Council of the Time Lords appoint the Valeyard as prosecutor at the Sixth Doctor's trial, hoping to have him executed and thereby remove the sole witness to their near-destruction of life on Earth.