The novelization of Star Wars: The Rise of Skywalker subtly restores an Expanded Universe Sith into the canon. The Star Wars novelizations are always eagerly anticipated; in the case of Star Wars: The Rise of Skywalker, there was a general hope it would resolve some of The Rise of Skywalker's many plot holes.
Naturally, all attention is on the return of the Emperor Palpatine, which went entirely unexplained in the movie itself. Rae Carson's novel reveals the resurrected Palpatine is actually a clone; after the Emperor's death in Return of the Jedi, the Sith Lord transferred his spirit into a clone body. Unfortunately for Darth Sidious, the sheer power of the dark side was too much for his new body, and it began to decay. He was only kept alive by Kaminoan nutrients, and his body was permanently attached to some sort of crane mechanism. The novelization describes this crane as an Ommin harness, and that's an important detail.
Ommin was an ancient Sith Lord introduced in the old Expanded Universe. The Star Wars Expanded Universe refers to the books and comics that populated much of Star Wars media between the release of Return of the Jedi and the prequels, and it was declared non-canon after Disney purchased Star Wars. Ommin played a major role in Tom Veitch's popular Tales of the Jedi: Knights of the Old Republic comics, which were set 4,000 years before the events of the first Star Wars film.
Ommin became king of the planet Onderon, and his obsessive studies of the dark side led to the accelerated decay of his body. He faded from visibility, allowing his people to believe him dead and his wife to rule in his place. The harness Ommin wore is visually identical to Palpatine's in Star Wars: The Rise of Skywalker.
It seems King Ommin is canon once again and that the Sith engineers of Exegol reproduced the harness he wore all those millennia ago. Interestingly enough, it's increasingly looking as though almost every element of Veitch's comics have been absorbed into the canon. Onderon itself appeared in Star Wars: The Clone Wars, the homeworld of Saw Gerrera, who has become a key figure in the Disney era. The dark side creatures created by the Sith on Onderon were canonized in Cavan Scott's audiobook Dooku: Jedi Lost, which subtly referenced those ancient wars.
Of course, there's a certain degree of irony to Lucasfilm drawing such a minor plot thread from one of Veitch's stories. Palpatine's return - indeed, the very idea of his spirit possessing a clone body - is lifted from the Dark Empire comics, which were also written by Veitch. It seems he had a surprisingly profound influence on Star Wars: The Rise of Skywalker.