Carnor Jax is a human Imperial Sovereign Protector — a member of the Emperor's Royal Guard given dark side training — and the antagonist of Dark Horse's Star Wars: Crimson Empire, a six-issue series by Mike Richardson and Randy Stradley published from December 1997 to May 1998.
Killing an immortal
The Dark Empire comics had established that Palpatine survived Endor by transferring into clone bodies. Crimson Empire's premise is the obvious next question: what if someone with access sabotaged the clones? Jax supervises the poisoning of the reserve bodies, ensuring that the next death is permanent, and then moves to take the throne himself. It is a coup carried out by maintenance.
The Guard as a subject
The series' real achievement is taking the red-robed background figures from Return of the Jedi and building an institution out of them — a brotherhood with a training world on Yinchorr, a code, and an internal politics. Jax has the massacre of his own order carried out to cover the coup, and the survivor Kir Kanos spends the rest of the series hunting him.
Continuity note
Jax is trained in the dark side and declares himself a Dark Lord of the Sith, but he is not part of any Rule of Two lineage and no Legends source treats him as one. He is a soldier with dark side instruction and an enormous opinion of himself, which is a distinct and rather more interesting thing.
Canon status
Star Wars Legends. Crimson Empire, Dark Empire and the cloned Emperor were moved to the Legends shelf in 2014, although the resurrected Palpatine returned to canon by a different route in The Rise of Skywalker.