Nidell was a Halanan female and the last wife of noted terraformer Gideon Seyetik. Her species have an unusual ability known as psychoprojective telepathy. Nidell lost control of hers, creating an alter ego, Fenna.
The Halanan project these alter egos when they are under great stress and can die from the experience. In order to save Nidell, Seyetik killed himself by flying a shuttle into a dead star and thus regenerating it. She recovered and did not remember her experiences as Fenna. (DS9: "Second Sight")