Star Wars has officially confirmed Sith spirits are in no way comparable to Force Ghosts. The idea of Force Ghosts runs all the way back to the first Star Wars film released in 1977, in which Obi-Wan Kenobi allowed himself to be cut down by Darth Vader. "If you strike me down," Kenobi warned, "I shall become more powerful than you can possibly imagine." He was proved correct, because he became Luke Skywalker's invaluable counselor from beyond the grave. The same, however, can not be said for fallen Sith dark lords.
Only a few Jedi have learned the secret of becoming Force Ghosts. But it is a power the Sith have long sought - albeit for very different reasons. Matt Stover's novelization of Revenge of the Sith - written in close coordination with George Lucas himself - stresses that their selfish motives ironically mean the Sith can never attain to true life after death. "The ultimate goal of the Sith, yet they can never achieve it," Qui-Gon explained. "It comes only by the release of self, not the exaltation of self. It comes through compassion, not greed. Love is the answer to the darkness." The Sith can, however, retain a degree of consciousness after death by binding their spirit to something else - a geographical place, an object, or even a person whose body they possess. Palpatine sought to use this technique, known as essence transfer, against his granddaughter Rey in Star Wars: The Rise of Skywalker. Star Wars has finally confirmed how different a Sith spirit is to a Force Ghost, however.
Lucasfilm has recently published an excerpt from Adam Christopher's upcoming novel Shadow of the Sith, a story set shortly before the sequel trilogy. In the excerpt, Luke Skywalker is mystically transported to the ancient Sith redoubt of Exegol, where he is confronted by nine Sith spirits - described in terms analogous to the Sith spirit of Darth Bane faced by Yoda in Star Wars: The Clone Wars. Although Luke knows he is battling "nine embodiments of the dark side," he is nonetheless shocked to realize these beings do not exist in the Force at all. As the novel states:
"The Force connected all life in the galaxy, but it also surrounded and penetrated the inanimate. Objects — rocks, planets, starships, droids, everything — had a presence in the Force, or rather, an absence that could be felt as strongly as if they were living things themselves.
But the wraiths were nothing. Luke couldn’t sense them with the Force at all."
This essentially confirms just how different Sith spirits are to the Force Ghosts immortalized in the Star Wars cinematic saga.
Luke's observation is a fascinating one, confirming once and for all that neither Darth Plagueis nor Palpatine were lying when they claimed the techniques they had mastered to preserve their lives were "unnatural." It seems Sith spirits like these nine beings do not really exist in the Force at all; they are not truly part of the real world, but rather are a presence from the netherworld impinging upon the realm of the living. It is possible the Sith spirits Luke battled were unusually strong, because they are on the planet Exegol - a vergence in the Force, a place where the veil between the plane of existence and death is unusually thin.
To his surprise, Luke is helped by the Force Ghost of Anakin Skywalker himself - proving a Force Ghost is also infinitely greater than any Sith spirit, because the wraiths flee from his very presence. This explains why the Sith envied the Jedi the possibility of becoming Force Ghosts; their own take on immortality is nothing more than a pale imitation, lacking in the absolute power the Sith seek. Once again Star Wars has revealed the balance of the galaxy - indeed, of creation itself - tilts towards the light.
