Xais (also known as Xais of Guaal, princess of the Guaal Territories, last of the Ugly Mutants or C-RAD, which stood for "cellular remission and decay") was a psychicmutant human. When she was young, Xais and her people lived in Guaal on the sixth planet of the Uva Beta Uva system. Miners came to the planet and killed all the mutants, so they would not interfere with their operations. Seeing everyone she loved being murdered unleashed her latent psychic powers. She mentally crushed all the miners in sight. Afterwards, she found a spaceship and went to the most populous planet in the system, New Earth (Uva Beta Uva Five). Blaming all “Normals” for the death of her fellow mutants, she killed two thousand people before she was 19. Xais finally was captured and brought to the Rock of Judgement, where she was tried and executed via particle reversal for her crimes.
Before her execution, though, artist Menlove Stokes made a mask of her face out of helicon. She was able to imprint her mind in the mask, allowing it to survive.
Three years later, she had gained enough strength to influence Margo to put on the mask. Now mobile, she continued her quest to destroy all “Normals”. She planned on mining a large amount of helicon on Planet Eleven, then imprinting it and using it to create an army of mental duplicates.
When Margo's body was killed, Pyerpoint forced the mask onto Romana II. Eventually, the Time Lady was able to overcome Xais' influence and remove the mask. Still needing a host, she convinced Pyerpoint to put on the mask so she could give him the secret of her immortality. While struggling for control, he was killed by one of the station's robots, which Pyerpoint had ironically programmed earlier to kill any intruders. Deprived of a body, she could not stop the primed helicon from expanding and covering the surface of Planet Eleven. The Fourth Doctor gave Frank Spiggot a formula which would break down the helicon, destroying what remained of Xais forever. (PROSE: The Romance of Crime, AUDIO: The Romance of Crime)