Xanxia was the queen of Zanak at some point. She was rumoured to have evil powers and the ability to live for hundreds of years. She staged galactic wars to demonstrate her powers, but ended up ruining the planet and there were few survivors. When the Captain crashed landed, she saved him by fitting him with cybernetic parts, which she used to control him.
At some point, Xanxia created a hologram of herself, with which she could pretend to be the Captain's nurse. She persuaded him to jump Zanak continually through space to plunder planets for rare minerals, and the energy required to feed the time dams.
Xanxia hoped to create a permanently youthful, viable body by preserving her true form in its last few seconds of life. The Fourth Doctor dismissed this as foolish, stating that there wasn't enough energy in the universe to keep the dams running forever. She would always be dependent on the last few seconds of life in her old body, and eventually she would die anyway. Xanxia was defeated by the Doctor when Zanak was prevented from materialising around Earth. The distraction caused by this failure allowed the time dams and the mountain in which they were enclosed to finally be destroyed by the Mentiads. (TV: The Pirate Planet)