Xoanon was a supercomputer meant to guide the Mordee expedition. The Doctor helped to repair it, but left behind his own personality, causing it to go mad with confusion.
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. They called him the Evil One. Ironically, both sides worshipped Xoanon.