Byss.
In the time of the Galactic Empire, Byss would be transformed into Palpatine's throne world once he became Emperor. It was perfectly suited for him after all, appearing to all the world to be a perfect place to live… but with a darkness lurking underneath, unbeknownst to its inhabitants.
Indeed, from the moment that Sidious had decided to make it his personal resort, his very own 'vacation' planet as well as his greatest stronghold, he had set out to remake the world in his image. Every time he visited, he would suffuse the planet itself with more of the Dark Side, until the very air was turned to his will.
The inhabitants, unaware of this, would live seemingly perfect lives filled with pageantry and entertainment, all the while having no idea that they were in turn feeding their life energies directly to Palpatine himself. Their lifespans would be noticeably shorter in any other location, but because they were so busy frivolously frolicking and being pampered and having the time of their lives, none of them ever realized that the turnover was something to be suspicious of.
By the time of the Galactic Empire, Byss was a fortress, a world hidden behind numerous protections. From Imperial Interdiction Fields to Mined Hyperspace Lanes cutting off all but one route to the planet, as well as its own personal fleet of Star Destroyers, it was quite frankly untouchable.
Right now, however, it was none of those things. Right now, Palpatine would have just begun construction of the Citadel that would one day tower over the rest of Byss, a testament to his vanity and power. And at the same time, he would still be moving things into place to have his own private supply of clone bodies to jump into in the event that he was somehow killed.
Learning about those had taken Vader quite a lot in the original timeline, truth be told. And by the time he did find out about them, he couldn't do anything either. Indeed, the only time Palpatine had allowed his apprentice onto Byss' surface had been early after the Clone Wars. There it had been revealed that a number of captured AgriCorps workers and Jedi Padawans were being held captive there, slowly being corrupted by the Dark Side and drained by Sidious.
Vader had killed all but four of them, still too angry for it to have even been considered a mercy. The final four had become the first of the Imperial Inquisition, Dark Side Acolytes who spread the Emperor's Will across the galaxy.
Unfortunately, that had been the last time he was allowed near Byss, so by the time he found out about Palpatine's clones, there wasn't anything he could do. But that was then and this was now. In this time period, Byss was not yet the fortress that Sidious would turn it into. However, it was still critical to his future plans… and now was the perfect time for a bit of sabotage.
In the future, the security surrounding Byss would be a combination of automated and biological, with the fleet of Star Destroyers eventually stationed there providing the biggest form of protection to the planet itself. But right now, the protection was purely automated.
This gave Vader the ability to slip right on through without anyone the wiser, simply reaching out through the Force to interface directly with the technology and make it register him as an acceptable visitor. Within the day, he's arrived over Byss by way of hyperspace with no one the wiser.
Landing on Byss itself is likewise just as easy. There's no fleet in orbit yet. There's no planetary shield to keep people out. Palpatine is not yet Emperor, so he has to move much more slowly than he would otherwise.
Landing near the half-constructed bones of the eventual Imperial Citadel, Vader leaves Artoo with the ship and begins making his way forward. Where the protection of the planet might be automated… the construction is not. It could have been quite easily, to be fair. Your average being would say that droid construction was much more efficient than construction by biological intelligence.
However, Darth Sidious was not your average being… nor did he care all that much about efficiency. No, what Sidious cared about most of all was propagating suffering. His Imperial Citadel needed to have the blood, sweat, and tears of slave labor baked into its very foundations in order to become the Dark Side Nexus that he needed it to be.
Seeing the thousands of non-human slaves laboring to build the monument to Palpatine's pride as he moves carefully along the edges of the half-constructed Citadel, Vader can't help but thin out his lips. In truth… he's already considering changing his plans.
Originally, he wasn't going to do anything that would let Palpatine know that something was wrong. Instead, he was aiming to infiltrate the nascent cloning lab that he knew was buried deep in the bowels of the half-constructed Citadel, where Kaminoan Scientists and Kaminoan Technology had been relocated in recent years to begin the analyzing of Palpatine's DNA and the creation of his supply of extra clone bodies.
From there, Vader had intended to introduce a minor alteration to the order for bodies, one that would theoretically go unnoticed but ultimately ruin this first batch of clones so that when he finally did kill Palpatine, the Sith Master's spirit would have nowhere to go.
However… if he simply did that and left, all of these slaves would continue to suffer and ultimately be killed in grisly, gruesome ways once their laborious work was done. If all went to plan, their deaths would not fuel Palpatine as he intended, because Palpatine himself would never have the chance to make use of this place. Vader would kill him first.
And yet… they would still suffer and die. Each and every one of them, forced to spend every single bit of strength and stamina they had left. Enslaved until the end of their days.
Vader's hands curl into fists at his sides and while he tightly controls it, he nevertheless feels an ice cold rage at the thought of it. Slavery was abhorrent. Even if he was a Sith Lord, he believed this with every fiber of his being. Among the many monstrous acts that he had committed in the future that never was, propping up slavery and perpetuating it as an institution across the galaxy were among his most regretted.
He would rather have killed Palpatine's enemies then enslaved them, but more often than not… he was not given a choice.
Now though, Vader did have a choice. In front of him were two paths… and ultimately, he could not be true to himself if he did not walk the right one.
With that said, he still makes his way into the depths beneath the bones of the half-constructed Imperial Citadel. He still slips into the hidden cloning laboratory tucked away down there, remaining invisible in the Force to both biological and droid eyes.
He finds what he's looking for easily enough, the mainframe for the cloning project accepting his dominion over it as easily as any other piece of technology. From there, he does precisely what he came here to do and inserts a malicious 'error' into Palpatine's DNA, ensuring that this first batch of clones will come out sickly, deformed, and ultimately dying before they're even taking their first breath.
None of the Kaminoans onsite even notice his presence. None of them will notice the sabotage either, Vader is sure of that.
However… once he makes his way back to the surface, he does not return to his ship. His original purpose is done… but that just means it's time for his other work to begin.
Making his way to the massive prison barracks set up for the slaves working on the Citadel, Vader arrives with purpose at the control center. There, men who are explicitly loyal to Sheev Palpatine above all else, are overseeing the slave labor that they're utilizing to carry out the Sith Lord's will.
Vader kills them all, slaughtering them with ease. There's only a couple of dozen in total. A couple of dozen to watch over thousands… but then, when those thousands are all one button press away from blowing up, you don't need many more than that now do you?
However, with the overseers dead, there is no one to press the buttons anymore. No one save for Vader himself. It takes mere minutes for him to permanently deactivate the explosive implants in every single slave currently on the planet. At the same time, he goes ahead and institutes a comms blackout as well, ensuring that anyone who does survive the coming revolt won't be able to send a message off planet either.
Then and only then does he take up a meditation pose in the middle of the control room and sink himself into the Dark Side of the Force. Reaching out, he connects to the slaves of Byss just as he connected to the Republic Fleets above Malastare weeks before. It's both easier and harder this time around. Easier because of the proximity, more difficult because these are slaves, not disciplined and well trained soldiers.
Still, it's not difficult for him to begin working them up into a frenzy. All over the construction site, there are more overseers hurling verbal abuse and in some cases physical abuse upon the slaves there. The remote controls they have that will allow them to detonate any slave on a whim keep them in power and keep the slaves in line.
Until that is, Vader pushes the first and angriest of said slaves over the edge. They turn on their overseers and begin to fight back, prompting the slavers to panic and try to activate the bombs.
Their remotes don't work, of course… and as more and more slaves turn, as Vader uses Battle Meditation to make them unknowingly coordinate with each other, more and more overseers find out that they are no longer the ones with the power. That they no longer have the authority they believed they did.
A satisfied smile comes to Vader's face as all across the half-constructed Citadel, slavers die and slaves free themselves from their chains, metaphorical and otherwise. The complete lack of any detonations baffles the slaves of course, but they don't bother to question it for long. They rise up and they kill until the killing is done… until not a single stinking slaver is left alive on the entire surface of the planet.
From there… well, what happens next is up to them. They can't escape. Byss is still protected by automated defenses and the smarter among the slaves knows that. Even if they do try to take one of the few transports off planet, they're never going to get away.
However… there's nothing saying they have to escape. Byss is a paradise world even now, and unlike in the future, Sidious' Dark Side Energies have not fully seeped into the planet yet. The Citadel was to be the nexus of his power, the center point of his authority. Now… now what has been made of it so far is being torn down by unruly, freed slaves.
Instead of the despair, suffering, and pain that Sidious thought to build his Imperial Citadel with, hope and freedom and other positive feelings permeate through the air and area.
The secret cloning lab deep beneath the ground remains untouched, with the Kaminoans onsite locking down the facility and trying to send out a distress signal that won't make it anywhere. However, the rest of the planet… that belongs to the freed slaves now. It is theirs to do with as they please.
Will Sidious eventually realize something has happened when his routine check-ins with Byss don't return any response? Yes, he certainly will. However, even if he does manage to hear about it and arrives to clean things up, he won't think to check if there's anything wrong with his clones because as far as he's concerned, the cloning lab was never discovered.
Vader can have the best of both worlds this way… and really, if all goes to plan, he doesn't intend to let Palpatine live long enough to realize what has happened anyways.
Regardless, with his work now done, Vader departs from the control center, leaving a slaughter behind for the freed slaves to eventually find when they gather enough courage to try and figure out what happened. It will leave them with more questions than answers, but to be fair… he gave them their freedom. He doesn't owe them anything more than that.
Making it back to his ship and slipping back out of the automated defenses and into hyperspace with ease, Vader rolls his shoulders, pleased with his progress so far.
… It was time to push forward other plans though. Time to send Ventress on to Dathomir so she could not only learn at the feet of her people but also seek an alliance with them on his behalf. Once that was secured and their plans for Padme were fully realized, it would finally be time to plan out the endgame…