Death comes for all; the Jedi and Sith are no exception. Yet some who master the Force seem to find ways to resist the call of eternity and exist as an essence of their former selves. In this form, they may speak with the living and guide--or manipulate--old friends and former enemies.
The most notable example in recorded Sith history is Darth Vax, a prominent member of the Dark Council. After her death, she was unable to let go of the power and influence she enjoyed during life. When her son took her vacant seat on the Council and assumed the title of Darth Arrid, she began manipulating him behind the scenes, making him a puppet for her own agenda. Four years passed before the Council realized the young Darth Arrid was only a mouthpiece for the spirit of his mother.