The Evil One was a figure from Sevateem religion.
A rock face came to bear a carving of the Doctor's face in tribute to this mythological character.
It came to be believed by the Sevateem that this was the Evil One, who kept their god Xoanon captive and had to be killed that he might be freed.
When the Doctor returned to the planet, they attempted to execute him to fulfil this prophecy. However, the Time Lord was able to extract his personality from Xoanon, prove that it was no god, thus rendering the Evil One negligible. (TV: The Face of Evil)