DOCTOR WHO IN DETAIL

The Valeyard is the darker side of the Doctor, between the twelfth and thirteenth regenerations. The High Council of the Time Lords, worried that the Doctor might find out their affairs on Earth/Ravalox, brought him into being.

The Time Lords offered the Valeyard the Doctor's remaining regenerations if he would kill his sixth incarnation. The Valeyard agreed, and a trial of the Doctor was arranged. The Valeyard tampered with Matrix evidence ensuring the Doctor's guilt.

With the aid of the Master, who wanted the Valeyard dead more than the Doctor, the scheme of the Valeyard's was uncovered and the High Council of Time Lords brought down.

After an explosion in the Matrix it was thought that the Valeyard had been destroyed, but he had killed, and taken the role, of the Keeper of the Matrix.

The seventh Doctor later battled the Valeyard in the guise of Jack the Ripper.