Xoanon

Xoanon was a supercomputer meant to guide the Mordee expedition. The Fourth Doctor helped to repair it, but left behind his own personality, causing it to go mad with confusion. Following that, it would sometimes project an image of the Doctor's face. Xoanon spoke using a variety of voices, including that of the Doctor.

Biography
The insane Xoanon created two tribes to represent his conflict: the more primitive and superstitious Sevateem (a corruption of "Survey Team") and the intellectual Tesh ("Technicians"), who used psychic powers and technology. He hoped to enact a war in which one of the tribes would destroy the other and show him which side of himself deserved to exist.

The Sevateem knew what the Doctor looked like, calling him the Evil One. Ironically, both sides worshipped Xoanon. (DW: The Face of Evil)

Behind the scenes

 * The various voices of Xoanan were supplied by Tom Baker, Pamela Salem, Rob Edwards, Anthony Frieze and Roy Herrick. At the time that they recorded their parts, Salem and Edwards were rehearsing for the following story DW: The Robots of Death, in which they played Lish Toos and Chub respectively.