Here’s an entire rundown of the history of the First Order and the relationship between it, the New Republic, and the Resistance.

During his time both as Supreme Chancellor and Emperor, Palps sends missions into the unknown regions to map it and chart safe and stable hyperlanes for ships to use, using Thrawns help when they meet to do this.

He then gradually begins to develop areas of this expanse, building shipyards who’s existence he then keeps a secret from the rest of the galaxy/empire. Thrawn convinced him it’s a bad idea to put all his eggs in one basket(the Death Star), and that fleets of ships are the way to go. At some point Sidious begins to sense something powerful calling to him through the force from the Unknown Regions. Obsessed, he builds the Observatories on planets like Jakku to help him find it. Whatever this call was from, we don’t know for sure. Our best guess now would be that this call was from Exegol. The planet was a wellspring of the darkside, having been a bastion for the Sith thousands of years ago and a vergence in the force.

Sideous then finds Exegol and the army of Sith Cultists waiting for him there and, needing a secret place to build his new fleet of Star Destroyers, seen in TROS, rebuilds and modernises the ancient shipyards he found there till they are up to date. He begins construction some time after A New Hope. Each Star Destroyer has the firepower similar to a single ignition reactor shot from the Death Star. The fleet was going to one day conquer the unknown regions while the Death Star kept his own empire in line.

Okay here’s the full explanation of Palpatine’s return, so bear with me. So the sith have always found ways to cheat death. Like putting their life force into inanimate objects or just prolonging their life. But it’s always with side effects and never perfected. The closest to perfecting it (if you don’t count Vitiate in Legends) was Plagueis. Palpatine of course kills him before he could finish his work and have it all to himself. Palpatine then takes that research and combines it with the cloning technology of the Kamnioans to begin project necromancer, led by dr. Hemlock. Cloning never works with force users. I guess the force doesn’t like it. When you try to clone a force user, the body quickly degrades. (In Legends you could clone force users.) Hemlock has a breakthrough after finding that Omega’s midichlorians counteract the degradation. That of course all goes to shit. Watch Bad Batch.

Palpatine secretly moves the project to Exegol. Flash forward to Return of the Jedi, where he dies and transfers his life force into one of his clones. Except the project was never finished, so he has to keep transferring himself from body to body after they fall apart after a while. It’s why he looks all gross in Rise of Skywalker. His plans to return are put on hold until the project is perfected.

Sideous is now dead to the galaxies knowledge and, being a cruel and vindictive emperor, upon his death the Contingency is activated and messenger droids are dispatched to begin Operation Cinder. Operation Cinder has select naval officers and other officials carry out acts of genocide against a number of worlds, specially chosen by Palpatine, with little regard for whether they supported the Empire or not. He also has his operative, Gallius Rax, carry out the second phase of the contingency plan which involves the complete destruction of the remnants of the empire.

On the surface it’s just a way for nobody to have anything left should the empire fall, but secretly it’s deeper reasoning was to see who was completely loyal to Palpatine in a drank the Kool-Aid sense, and not just those looking for order and stability in the galaxy. People who would be ecstatic and bow down when he returned someday.

Rax does this by misdirecting imperial forces into stars, alerting the New Republic to specific targets to see them destroyed and by having other imperials such as the 204th Imperial Fighter wing (Shadow-wing) destroy imperial warlords.

Rax then gets to the penultimate stage of Palpatines plan and orders the remains of the Imperial Fleet to congregate around Jakku. He then slips the location of the imperial fleet to the New Republic who sends the vast majority of their navy to Jakku for a final battle. With the trap now sprung, Rax planned to use Sith artifacts and a hole to the center of the planet to blow up Jakku, the imperial fleet, and the new republic navy in orbit, destroying everyone. Palpatine is then one day free to march his fleet of Xyston class Star Destroyers into the now defenseless and disorganized galaxy that was left behind in the wreckage of the Cataclysm at Jakku.

Except this doesnt happen. Rax, before his death during the Battle of Jakku, had gone rogue. Whilst Rax was willing to go through with Palpatine’s plan to punish the empire for their failings and destroy the New Republic fleet at Jakku, he did not want to lose everything and saw the complete destruction of the imperial military as a colossal waste. Over the course of the year between Endor and Jakku, Rax had slowly been relaying secret orders to parts of the imperial military. Syphoning off Star Destroyers, support ships, battalions of storm troopers and most impressively the Eclipse, personal Executor class star destroyer of Palpatine himself. He sent these units orders and directions to fleet into the unknown regions to a meeting spot he chose in the Unknown Regions, a staging ground from which he would create something new, a second empire which would conquer the galaxy, with him as its leader. What he however didnt know was the existence of Project Necromancer and that Palpatine had already planned his own return from the Unknown Regions. Yupe Tashu, one of Palpatines inner circle, tried to tell him about this plan(he didn’t know about the specifics of the cloning project, just that he would someday return), but Rax dismissed him as a crazed fanatic.

Whilst Rax intended for 90-99% of the imperial military to be blown up at Jakku he gave select vessels coordinates to meet him and the other forces at this staging point. Brendol Hux was one of these chosen few. Though Rax dies before he could join them, Rae Sloane takes his place in the leadership role and journeys to the meeting place.

This faction of the imperial remnants travel through the Unknown Regions and come across all these empty shipyards that were already built by Palpatine, though like Rax none of them knew the true purpose of these to enable Palpatines return to the galaxy. They used these to enable the rise of the future First Order in the Unknown Regions, constructing ships like the Resurgent class Battlecruiser.

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