Sith Eternal

The Sith Eternal was a cult of Sith loyalists that worshipped the Sith Lords of the galaxy, serving them as faithful lackeys and disciples. They appear in Star Wars: The Rise of Skywalker.

Background
The Sith Eternal operated throughout Galactic history, but by the time of the New Republic had been operating in secret, seeking to revive the traditions of the Sith as well as the Sith Lords themselves after Palpatine and Darth Vader had been destroyed. They were the main force operating behind the creation of the First Order, the bulk of the military force known as the Final Order, and they were spread throughout the galaxy. Their true base however, was the hidden world of Exegol, where the bulk of their forces were stationed. They could also speak the Sith Language fluently.

Organization
Though the chain of command in the Sith Eternal was unknown, they were all worshippers of the Dark side of the Force. They focused both on spiritual and military matters, as was traditional for their cult. It is known that engineers, ship builders and slaves they used as labor existed in their ranks. They were also steeped in rituals created 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 Palpatine from death.

Military
The Sith Eternal served as the hidden heart of Palpatine's plans to reconquer the galaxy while the First Order served as the military arm of his operations. In spite of this, the Sith Eternal's military might was terrifying. The Sith Eternal's ground troops consisted of thousands of Sith Troopers, clad in red armor. Their military was organized according to ancient Sith standards; legions of 5,000 men and within those, squads consisting of 10 men led by seargents divided into three triads consisting of lance corporals leading two corporals. Each legion was 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 were likewise named after Sith weapons; examples included "Lanvarok Squad", "Parang Squad", and "Warblade Squad".

After the Sith Eternal absorbed the First Order into its ranks and took more direct control, friction occurred as the First Order became the Final Order, and the pragmatic First Order officers clashed with the fanaticism of the Sith Eternal's officers and cultists.

Equipment
The Sith Eternal's equipment and weapons were identical to that of the First Order, and their Star Destroyers, which resembled the Galactic Empire's, had miniaturized planet-destroying cannons similar to the Death Star's.

Star Wars: The Rise of Skywalker
The Sith Eternal remained hidden throughout the Galaxy for most of its history. After the events of the Battle of Endor, they relocated to 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 Star Destroyers and created an army of Sith Troopers in preparation for a renewed conquest of the galaxy.

During the age of the New Republic, the assassin Ochi was sent to hunt down Palpatine's family, seeking to find them and shape their child, Rey, into a suitable heir to the Sith throne by the Sith Eternal. Ochi failed as she had been left on Jakku for protection. The refusal of her parents to reveal her location caused Ochi to murder them, and he himself was later killed on the planet Pasaana.

At some unknown point, using both the Dark side of the Force and technology they invented, the Sith Eternal successfully resurrected Palpatine, partially reviving him, though he was forced to remain attached to life support systems due to his undead nature. Seeking an heir to the Sith legacy, Palpatine 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 succeeded at when he successfully killed Snoke by way of deciept 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 Palpatine's survival, the Sith Eternal gave Kylo Ren the location to a Sith Wayfinder, a device that would lead him to Exegol. Upon his arrival, Palpatine 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. Learning that she was Palpatine's granddaughter, Kylo ren sought to make her his apprentice. In spite of the Sith Eternal's choosing of him and giving him their vast empire and resources, Ren felt no loyalty to them and felt no desire to be subservient to their wishes or to Palpatine.

Ultimately, Kylo Ren returned to being Ben Solo after his mother, Leia Organa died. However, Rey, being Palpatine's direct descendant, was chosen as the heir to the Sith instead. Rey journeyed to Exegol, encouraged by Luke Skywalker to face her fears, using the Sith Wayfinder stolen from Ben, and confronted Palpatine. Palpatine 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 complete her journey to the Dark side to provide the galaxy with a worthy Sith ruler. To coerce her into fulfilling his wishes, the Sith Lord sent a Star Destroyer 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 and the Final Order 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 Palpatine. Using the power of their Force Bond, Palpatine drained them to restore himself completely to life. Resurrected, Palpatine decided that he alone would rule and 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 Palpatine, blocking his Force Lightning with Luke and Leia's lightsabers. Redirecting his attacks, the young Jedi destroyed her grandfather, with the resulting explosion killing his followers watching the duel. The destruction of the Final Order, combined with the destruction of the cultists in Palpatine's temple on Exegol, destroyed both the Sith and the Sith Eternal once and for all, freeing the galaxy from their tyranny forever.

Trivia

 * 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 (Palpatine and Marka Ragnos, respectively), but the Disciples of Ragnos were all Force-Sensitive warriors, while the Sith Eternal were not.