A Dalek duplicate of Nyssa was created on the planet Mojox based on DNA samples acquired by the Daleks after they briefly captured the Fifth Doctor and Nyssa. While the two were able to escape captivity shortly after they reached the planet, the Doctor departed without realizing the scale of the Daleks' plans for the planet.
This duplicate of Nyssa was assigned to be the Daleks' Chief Virologist, using Mojaxalli test subjects to perfect a virus for the Daleks that would be used to turn the Mojaxalli population against one another in violent rages. The first duplicate was contacted by Toru, who showed her footage of the real Doctor and Nyssa's escape and lead her to a facility full of duplicates of her and the Doctor when the duplicate Doctor and the Daleks arrived and killed her.
A second Nyssa duplicate was provided the same information and abducted the Doctor, bringing him to the facility to discover the truth of their existences and simultaneously sabotaged her viral research so that the final result was a virus that would drive the Daleks to destroy themselves rather than the Mojoxalli, which also destroyed herself and the duplicate Doctor, who had been infused with Dalek DNA. (AUDIO: Dalek Soul)
Personality
Equal with her template, the duplicate Nyssa was a brilliant biologist and, while having no compunctions with the use of live test subjects, had a sense of guilt over the fates of the Mojaxalli who she experimented on but said nothing so as to not invite the Daleks' wrath. She was skeptical after witnessing video footage of the original Doctor and Nyssa, but her scientific nature lead her to the truth. After discovering the truth of her existence, she demonstrated traits of the real Nyssa, hoping to live up to the Doctor's standards. (AUDIO: Dalek Soul)