-- SPOILERS lie ahead for The Last Jedi --
Concept art from Star Wars: The Last Jedi reveals that Sith Force Ghosts nearly made an appearance in the movie. It's not as if Episode VIII was lacking for shocking moments. From the controversial deaths of leading characters to dramatic plot twists, TLJ had more than its share of jaw-droppers. Director Rian Johnson's film defied expectations in pretty much every way imaginable, disposing of the new trilogy's presumed main villain before later killing off the franchise's greatest hero. Nearly every fan theory went up in flames, and a ton of fanboys were pissed off along the way.
One of the greatest surprises featured in The Last Jedi was a Force Ghost appearance by Jedi Master Yoda. As you know, the spirit of Luke Skywalker's former mentor popped up on Ahch-To about halfway through the film to share some wisdom and help set him on the right path. But as it turns out, what we saw onscreen only scratched the surface of the director's vision. At one point, Johnson considered including Obi-Wan Kenobi in the Force Ghost scene. And according to a recently released collection of concept art, Force Ghosts were almost taken even further than that.
The Art of Star Wars: The Last Jedi has revealed that the universe's spirit realm may not be as exclusive to the good guys as we thought. It would seem that at some point during production, an actual Sith Force Ghost was on the table. Judging from the artwork below, plans were in place for Luke to be visited by a distinctly evil looking being.
Johnson has spoken previously about the potential for dark side Force Ghosts, but the caption provided in The Art of The Last Jedi is far more telling:
We've seen dead Jedi come back as blue ghosts. Maybe Sith can come back. And maybe there's some all powerful Sith that's controlling whatever the dark side is. We did talk a lot about how the final battle frontier for Jedi might be in the spirit realm. So you have to have a bad-guy ghost.
Judging from the comments on the Reddit post, this would not have gone over super well with the diehard fanbase, as it would have broken with many accepted beliefs of what a Sith is. As for who the mysterious figure hanging out in the background is, our best guess would be Darth Plagueis. Darth Sidious (Emperor Palpatine) would make more sense, since Luke had actually met him, but hey, it's concept art. It doesn't have to make complete sense.
Source: Reddit, The Art of Star Wars: The Last Jedi