Sith Citadel

The Sith Citadel, also called the Citadel, was an ancient Sith temple located on the hidden Sith world of Exegol. It was the headquarters of the Sith Eternal cult following the demise of the last two Sith Lords at the Battle of Endor and contained the throne of the Sith, which the resurrected Dark Lord of the Sith, Darth Sidious, intended to to serve as the seat of his new Empire.

Early history
The Citadel was likely built thousands of years before the fall of the Galactic Republic by the ancient Sith or Sith Loyalists, in order to honor Sith of the past, as well as have meetings and practice rituals in secret (as evidenced by the throne room amphitheater in the Citadel).

It might have been built on a vergence in the Force that ancient Sith Loyalists had been trying to find by means of excavating Exegol. This might explain why the Citadel appears to float a few meters above Exegol’s barren surface.

Palpatine’s Discovery
At some point in time before 19 BBY, Darth Sidious (before he became the Emperor) discovered Exegol and the Citadel on it with the help of a Wayfinder. Sidious likely occupied the Citadel for a time, learning the secrets of the ancient Sith and meeting with Sith Loyalists. He then used the Dark Side of the Force and genetic engineering technology in the Citadel to make Snoke, a humanoid being powerful in the Dark Side. He sent Snoke from the Citadel into the Unknown Regions of the Galaxy, where he would witness the rise and fall of the Galactic Empire.

Darth Vader likely discovered the Citadel/Exegol as well, sometime after 19 BBY, for he also had a Wayfinder. It is unknown whether he traveled there to learn more about the Sith or help Sidious (now Emperor Palpatine) with his projects.

Formation of the First Order
Some time after the Battle of Endor, in which Emperor Palpatine was killed by Anakin Skywalker, the former was brought back to some semblance of life by the Sith Eternal, who were occupying the Citadel at the time. Palpatine, who was physically very weak and was supported by many medical machines in the Citadel, manipulated events in the Galaxy so that Snoke would become the Supreme Leader of a growing faction of former Imperials, the First Order.

Palpatine also spoke to Ben Solo from the Citadel, pretending to be Snoke and Darth Vader, so that Ben would fall to the Dark Side. He might be responsible for destroying Luke’s Jedi Temple as well, by making some sort of Force storm. He might have used the Dark Side energies in the Citadel and Exegol in order to achieve this.

Appearances

 * Star Wars: Episode IX The Rise of Skywalker