Nephthys was the sister and wife of Sutekh. Since she was thought far more dangerous than her husband, while he was trapped in a pyramid on Earth by the Eye of Horus, her mind was split up into two parts: her instincts were stored in Sadan Rassul's daughter who was mummified alive, and her reasoning was stored in a canopic jar.
Her reasoning was released in 2000 BCE when robbers were plundering her pyramid and broke the canopic jar in which it was stored. In an attempt to escape her prison, she made arrangements to have her reasoning transferred into the mind of Nyssa, believing that Nyssa's mind would be easier for her to escape from. Her power and instinct were transferred into a clone of Rassul's daughter, Rassul hoping that the restoration of Nephthys's mind would also restore his daughter in her original body. The Fifth Doctor and Tegan were able to defeat this plan by switching Nyssa with her double, Lady Ann Cranleigh, tricking Nephthys into believing that Nyssa had "woken up" in 1926 and the aspect of Nephthys that she possessed had been released at that point. Acting on instinct, Nephthys travelled rapidly back and forth between 1926 and 1996, her lack of reasoning leaving her unable to realise what she was doing to herself until she aged to death. (PROSE: The Sands of Time)
Behind the scenes
In Egyptian mythology, Nephthys was one of the Egyptian goddesses, a guardian over the dead, alongside Isis, who guards the canopic jar containing the lungs.