She was the leader of the Fifth Planet Gambit, an expedition to break the time loop surrounding Planet 5 about 12 million BC, so the Fendahl could be released and used against the Enemy. Like all members of the expedition, she had been changed into a Elder Thing as camouflage during the time period.
In actuality, the mission had been diverted by the hermit. He feared the Swimmers would be attracted by the presence of Mictlan and destroy our universe. Somehow, he knew that the Fendahl had already been devoured by the Fendahl Predator, which could be used to destroy Mictlan. After the mission succeeded and broke the time loop, the hermit's agent One let Xenaria and some of the rest of the mission live so they could return to Gallifrey and tell them what had really happened. (PROSE: The Taking of Planet 5)