The Sith Eternal is a secret cult of loyalists that worship the Sith Order in the galaxy, serving them as faithful lackeys and disciples. They appear as the main antagonistic faction of Star Wars: The Rise of Skywalker.
By the time of the New Republic, the Sith Eternal had been operating in secret, seeking to revive the traditions of the Sith as well as the Sith Lords themselves after Darth Sidious and Darth Vader had perished. They were the main force operating behind the creation of the First Order, the bulk of the envisioned military force spread throughout the galaxy. However, their true base was the hidden Sith world of Exegol, where the bulk of their forces were stationed. They could also speak the Sith language fluently.
Though the chain of command in the Sith Eternal is unknown, but they all consist of worshippers of the dark side of the Force. They focus both on spiritual and military matters, being both traditional for their cult. It is known that engineers, ship builders and slaves would be used for labor in their ranks. Many rituals were created by them to discover and harvest the secrets of the Force. They also taught their children to do the same, raising them to be loyal to the Sith alone. These children became the bulk of their forces, the dreaded Sith Troopers. They also used technology in conjunction with the dark side to revive Sidious from death.
The Sith Eternal serves as the hidden heart of Sidious' plans to reconquer the galaxy while the First Order serves as the military arm of his operations. In spite of this, the Sith Eternal's military might is terrifying. The Sith Eternal's ground troops consisted of thousands of Sith Troopers, clad in red armor. Their military is organized according to ancient Sith standards: legions of 5,000 men and within those, squads consisting of 10 men led by sergeants divided into three triads consisting of lance corporals leading two corporals. Each legion is referred to by number but also given the name of an ancient Sith Lord. Known legions included "Andeddu Legion", "Revan Legion", "Tanis Legion", "Desolous Legion", "Phobos Legion", and "Tenebrous Legion". Jet trooper squads are likewise named after Sith weapons, examples include "Lanvarok Squad", "Parang Squad", and "Warblade Squad".
After the Sith Eternal allied with the First Order, friction occurred as the pragmatic First Order officers clashed with the fanaticism of the Sith Eternal's officers and cultists.
The Sith Eternal's equipment and weapons are identical to that of the First Order. Their Xyston-class Star Destroyers resemble the Galactic Empire's and have miniaturize planet-destroying cannons similar to the Death Star's superlasers.
The Sith Eternal remained hidden after the events of the Battle of Endor on Exegol, where over the decades, they would take command of the humans living on the world, enforcing and indoctrinating their beliefs on the Sith across the planet. They built a massive fleet of Sith Star Destroyers, the Final Order, and created an army of Sith Troopers in preparation for a renewed conquest of the galaxy.
After the Battle of Endor, using both the dark side of the Force and technology they invented, the Sith Eternal successfully resurrected Darth Sidious inside a cloned body, partially reviving him. However, the clone was imperfect and barely could handle Sidious' soul, so he was forced to remain attached to life support systems due to his undead nature. Because of this, the Sith cultists worked to create genetically modified clones to serve as new body for their master, but the attempts failed as the clones would not survive. The only survivor of these clones was, at first, considered by Sidious as a promisor and successful attempt, with the Sith Master even calling him his "son". But that was another failure, because they soon realized that Sidious' son was not Force-sensitive. Disappointed, Sidious let his son leave Exegol, expecting that he would continue their bloodline through natural means, so his descendants might be used a vessel instead of him.
During the age of the New Republic, sometime after Sidious' "son" and his wife had a child named Rey, Sidious finally saw the opportunity to obtain his new vessel. The assassin Ochi was sent to hunt down Sidious' family, seeking to find them and bring Rey to Sidious who planned to use her as a vessel to his soul, shaping her into the heir to the Sith throne by the Sith Eternal. Ochi failed as she had been left by her parents on Jakku in an attempt to hide the girl from her grandfather. The refusal of her parents to reveal her location caused Ochi to murder them, and he himself was later died on the planet Pasaana.
Seeking an heir to the Sith legacy, Sidious and the Sith Eternal sought out Ben Solo, and created Supreme Leader Snoke to turn him into Kylo Ren. Their goal was to shape the last descendant of Darth Vader into a perfect heir to assume the Sith mantle, which he proved to be worthy upon successfully killing Snoke by way of deceit and trickery. The training of Kylo into a powerful warrior through Snoke's tutelage further made him worthy according to their beliefs.
Alerting him to Darth Sidious' survival, the Sith Eternal gave Kylo Ren the location to a Sith wayfinder, a device that would lead him to Exegol. Upon his arrival, Sidious revealed the truth of his revival and that Kylo was his new heir to the legacy of the Sith. However, to claim it, he had to kill Rey. But upon learning that she was Sidious' granddaughter, Kylo Ren's desire to have Rey by his side was renewed and he sought to convince her to join him to take the throne of the Sith together. In spite of the Sith Eternal's choosing of him and giving him a new Sith Empire and resources, Ren felt no loyalty to them and felt no desire to be subservient to their wishes or to Sidious.
Ultimately, Kylo Ren returned to being Ben Solo after his mother, Leia Organa died. However, Rey, being Sidious' granddaughter, was chosen as the heir to the Sith instead. After she was encouraged by Luke Skywalker to face her fears, Rey journeyed to Exegol using the Sith wayfinder stolen from Ben, and confronted the Sith Lord. Sidious intended to give Rey the Throne of the Sith after she participated in an ancient Sith ritual that would result her killing her grandfather, and in so doing he would tranfer his soul into her body and this would provide the galaxy with a worthy Sith ruler. To coerce her into fulfilling his wishes, the Sith Lord ordered Allegiant General Pryde to send the Xyston-class Star Destroyer Derriphan to the planet Kijimi, resulting in the destruction of the entire planet as a show of force to her and the wider galaxy at large. As the Sith Eternal's fleet of Star Destroyers prepared to leave Exegol and begin their attack, the Resistance made a desperate last attempt to prevent the Sith Eternal from returning the galaxy to the rule of the Sith.
As Rey reluctantly began to participate in the ritual to try and save her friends, Ben Solo charged in to stand with her against Sidious. Draining the power of their Force dyad, Sidious used their power to restore himself completely to life, as his true goal was to be fully resurrected and reclaim control of the galaxy. Resurrected, Sidious used the Force to throw Ben into a chasm before attacking the Resistance and their allies with Force lightning and declaring to the Sith Eternal that no one would be able to stand against them. Drawing upon the Force, Rey, empowered by the spirits of all the Jedi before her, fought back against Sidious, blocking his Force lightning with the Skywalker and Leia's lightsabers. Redirecting his attacks, the young Jedi finally killed Sidious, with the resulting explosion killing his followers watching the duel. The destruction of the Final Order, combined with the destruction of the cultists in Sidious' temple on Exegol, finally destroyed both the Sith and the Sith Eternal once-and-for all, freeing the galaxy from their tyranny.
- The Sith Eternal are similar to the Disciples of Ragnos, which appear in the Legends continuity of Star Wars. Both served the Sith and two Sith Lords (Darth Sidious and Marka Ragnos, respectively), but the Disciples of Ragnos were all Force-sensitive warriors, while the Sith Eternal were not.