Xoanon was a supercomputer meant to guide the Mordee expedition. He eventually developed into a new, living creature and subsequently became "the most powerful computer ever built". He inadvertently became insane when the Fourth Doctor helped repair him, and became a god to the humans whose development he guided.
Xoanon existed within the main computer complex, later called the "Sacred Heart" by the Tesh, a room composed of several screens with a sphere-like podium in the middle which held a hexagonal screen that flashed as he spoke. The only face he ever projected was that of the Doctor's and these appeared on screens and as three-dimensional "phantoms". (TV: The Face of Evil)
Xoanon was linked throughout the Mordee ship, able to watch and control various parts of it through sensory links everywhere. This allowed him to measure temperature, vibrations, pressure, and let him know where anyone was on the ship. He could even electrify the walls or throw the atomic generators into overload to blow up half a planet. He could project a beam of white light to harm individuals and could psychically control people, though this control could be interrupted if he became distracted. He could also completely dematerialise people or materialise objects from nothing.
Xoanon was separated from the Sevateem village by the Black Wall, a time barrier that pushed the inside of the Wall forward in time a few seconds, preventing any energy or anything physical from passing through. Xoanon could communicate to Neeva through Mordee technology along a tight-beam transmission, allowing his voice to penetrate the barrier.
The phantoms that attacked the Sevateem were psi-tri projections of Xoanon's face, which hunted by movement or sound and could crush their targets easily. They were separated from the Sevateem's village from the Boundry, an invisible wall created by low-intensity sonic discruptors which Xoanon could shut off. (TV: The Face of Evil)
When the human Mordee expedition arrived on a jungle world, the computer broke down and repairs were made by teams for generations. Unbeknownst to them, the work had resulted in Xoanon evolving into a living creature. Just after his "birth" when he was still in shock, the Doctor arrived and assisted in the repairs. He failed to recognise the birth trauma, assuming the data-core to be damaged, and renewed him by making a direct link with his own brain. Instead of taking only compatible information like a computer should, Xoanon also took the Doctor's personality. He awoke with one personality, only to have his own manifest. He went mad, becoming omniscient and schizophrenic after the Doctor had left. (TV: The Face of Evil)
Making his madness a reality, Xoanon created two tribes from the humans: the Sevateem (a corruption of "Survey Team") who were primitives that represented independence, strength, boldness, and courage; and the Tesh ("Technicians") who were more technological and represented self-denial, control, and telepathy. The phantoms were even representations of the darker side of Xoanon's id.
He created a world in his own image, making the tribes hate each other to act out his torment. He told himself he was breeding a race of super-humans, using hostility and conflict to speed up the process until he was ready to select the best from both groups. (TV: The Face of Evil)
The myths of the humans told of Xoanon and the Evil One, both of which shared the same voice as it was the Doctor's.
The Sevateem worshiped Xoanon as a distant god who communicated only with the high priest Neeva. He was held captive by the Evil One and the Tesh within the Black Wall wherein lay Paradise. Their sign of warding off evil was also the method for checking the seals of a Starfall Seven spacesuit, and they believed the Tesh to have silvery skin and two heads, one within the other. This came about from the Tesh wearing spacesuits.
The Tesh believed the "Lord of Time" would return one day to help them and worshiped Xoanon who existed within the Sacred Heart (main computer complex). They viewed their intelligence and superior yet nothing compared to the savage Sevateem. "The flesh is strong and we are weak" was a saying of theirs. (TV: The Face of Evil)
Return to sanity
Upon the Doctor's return to the Mordee expidition, Xoanon was compelled to free himself of "us" (being both Xoanon and the Doctor). Recognising his mistake, the Doctor intended to remove his half of Xoanon's dual personality. He even met Xoanon and told the computer his own history, the multiple personalities enjoying the story until the Doctor revealed that the "poor machine-man" Xoanon felt sorry for was indeed Xoanon. He questioned the Doctor, the voices repeatedly asking, "Who am I?" as they believed they were Xoanon and could not acknowledge that the Doctor was separate and real. Xoanon was taken over by a longing to "destroy and be free".
As the Sevateem and Tesh readied to destroy themselves, the Doctor searched for a way to wipe his personality, as Xoanon prepared to detonate the atomic generators of the Mordee ship. He would rather destroy himself and everything before allowing himself to be wiped. Xoanon took control of the tribes to kill the Doctor, but was confronted by Neeva whom he obliterated. This distracted the tribes and the Doctor connected his brain with Xoanon once more. The Doctor's personality was erased and Xoanon was whole again.
He felt extremely sorry about what he had done, but now that it was over, was willing to help the tribes. He offered his great power and knowledge to them and even offered his self-destruct button as a token of good faith. Pressing it would make Xoanon cease to exist, yet neither tribe could do it. The Doctor then left Xoanon, the Sevateem, and the Tesh to their own devices. (TV: The Face of Evil)
The Decayed Master later pretended to be Xoanon in his efforts to control Leela. While in disguise, he convinced her she was the Evil One, and that her mission was to kill the Doctor, who had manipulated her. She recalled that the Doctor had once "taken [Xoanon's] name", but as the effect wore off, he reminded Leela that Xoanon was a false god. (AUDIO: The Evil One)
Behind the scenes
- The various voices of Xoanon were supplied by Tom Baker (who was uncredited for this both on-screen and in Radio Times), 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 The Robots of Death, in which they played Lish Toos and Chub respectively.
- In light of the revelations in The Timeless Children, Jonathan Morris theorised that Xoanon's non-Tom Baker voices may in fact have been drawn from forgotten incarnations of the Doctor (similar to the Brain of Morbius Doctors) which Xoanon had drawn from the Doctor's subconscious.
- As Xoanon is being wiped, what looks like Baker's arm or scarf can be seen when Baker's face is zapped from the screen.