The problem, as always, boils down to secrecy and the need for it. There will come a moment where Vader no longer fears discovery and every action he takes, every move he makes, is focused on reaching that point all the sooner.
Right now though… if his true alignment is discovered, either by the Jedi Order or Darth Sidious, then everything will get far messier than it needs to be. He still fully believes he would pull out a win, but it would not be without losses… and Vader wasn't going to waste this second chance on a pyrrhic victory of all things.
That said, there were two ways to go about checking the Jedi High Council for another Fallen Jedi Master. One approach would cast a wide net, while one would be akin to spearfishing. The problem, of course, was that trying to cast a wide net by himself would be… less than useful.
As things stood, the Dark Side already blanketed Coruscant and the Jedi Temple. The only way for Vader to possibly be able to see past Sidious' Dark Presence, let alone his own Dark Presence in a broad sense would be to utilize the Force Nexus hidden deep, deep beneath the Jedi Temple.
Once a neutral vergence in the Force of great power, the original Jedi Order had built their temple atop it back before Coruscant became the city-planet it was today. However, as the millennia had gone by, the Jedi had forgotten their roots… literally.
It was actually Sidious himself who rediscovered the Force Nexus beneath the Temple, and slowly but surely corrupted it to the Dark Side of the Force. This made it the perfect tool for Vader to use now if he wanted to do an entire sweep of the Temple and the Jedi Order to be able to see exactly who among the High Council had actually fallen to the Dark Side, or if any of them were close to falling to the Dark Side.
The problem with that was, Sidious was so in tune with the vergence that the likelihood of him detecting Vader using it was too damn high to risk. But without the Nexus, Vader couldn't just 'scan' the Jedi in the Force, they would almost certainly detect him instead.
In the end, his only real option was to act with a scalpel. Rather than a scattershot approach, his best bet was to pick his target and call his shot. Basically, go in assuming he already knew exactly which High Council Member had Fallen and then check them specifically to see if he was right.
The only question then became… who to check? He could check multiple, it would only take him time obviously, but where to even begin? Who among the Jedi High Council (aside from Shaak Ti obviously) would be tempted by the Dark Side of the Force?
Well, at least some of them were currently off planet, so that did narrow it down a little bit. Not that Vader would have considered someone like say, Obi-Wan, as the possibly culprit for even a second. Still… of the High Council Members currently on planet, he should be able to narrow the list down even further.
Grandmaster Yoda, for instance, was right out. Not only would the little gremlin almost certainly detect Vader's probe, the universe itself would end before Yoda fell to the Dark Side.
… Perhaps Master Windu though? Everyone knew that the Master of the Order utilized a form of lightsaber combat he himself had created with the help of Jedi Master Sora Bulq. Called Vaapad, it effectively allowed Windu to channel the inherent darkness within himself into a more 'positive' outlet.
It also made him especially efficient fighting Dark Side users, as Vader had seen in the original timeline when Sidious nearly died to the Jedi Master before manipulating Vader into saving his life. Still, Vaapad was like a lightsaber without a hilt. Not all of its users had managed to ride the edge of the darkness without being subsequently swallowed up by it.
But he didn't seriously think that Windu had Fallen. That would be almost as ridiculous as Yoda falling to the Dark Side, not least because of just how much time the Master of the Order and the Grandmaster spent with one another. If Windu had truly Fallen, Yoda would know about it within days.
So not him either. And Vader is quick to discard Ki-Adi-Mundi as well. If Mundi were going to Fall, he would have done it for love ages ago given he was one of the few Jedi allowed to have a family.
Hm, perhaps one of the newer members then? Adi Gallia had only recently been promoted to the Jedi High Council following the Battle of Geonosis. Before that promotion, she had already been the Jedi Order's diplomatic liaison with the Galactic Senate, having to rub shoulders and make nice with the corrupt politicians that infested the Senate Chambers. That role had not ended with her ascension to the Jedi High Council though. Perhaps she was overwhelmed by the increase in responsibility and duties. Perhaps such close proximity to Sidious had gotten to her at a particularly weak moment.
… No, Vader could check her later, but he didn't think she was worth checking first. Especially since she wasn't in the Jedi Temple right now anyways. She was off at the Senate and it would be easier to carefully probe her Force Presence once she was close by.
As he steadily makes his way to the Room of a Thousand Fountains, Vader finds his thoughts circling back around again. Specifically, to the creator of Vaapad. He still doesn't think Windu could have Fallen, but he finds himself returning all the same to the lightsaber style. Vaapad was a dangerous weapon, something that very few had ever learned.
One of those people just so happened to fit all of Vader's criteria as well. Depa Billaba was originally Mace Windu's apprentice, and she had of course inherited her Master's lightsaber style in the process. She had also only recently been promoted to the Jedi High Council to replace one of the previous Council Members lost on Geonosis.
On top of all of that, she had a history of falling to the Dark Side in the original timeline. Her final mission in the midst of the Clone Wars the first time around had taken her to Haruun Kal, which just so happened to be her Master's homeworld.
Vader didn't know exactly what happened there, only that it resulted in Depa Billaba's fall to the Dark Side and eventually her defeat at the hands of Windu himself. At which point she'd fallen into a coma that had lasted all the way to the end of the Clone Wars, where she'd been subsequently killed in the destruction of the Jedi Temple.
He'd done the deed himself, quick and easy, after dealing with Depa's grandmaster and Windu's Master, the Neti Jedi known as T'ra Saa.
Now though… well, if Vader was going to start anywhere, he might as well start with Depa. Probing her might not turn up anything, but that just meant he moved onto the next most likely target on his list.
Luckily, the Room of a Thousand Fountains also has about a thousand secluded meditation spots. It's not difficult for Vader to find a place within the seven-story greenhouse where he knows he won't be disturbed. Settling down into a meditative pose, he focuses first on masking his own presence. Hiding himself has already become relatively second nature since arriving in his timeline, but it wouldn't do for him to have a lapse during the probe.
Once he's certain of his own security, Vader closes his eyes and slips out into the hustle and bustle of the Jedi Temple. He isn't reaching out so much as he's slipping through the mass of metaphysical presence in the Force. What he sees from that cursory glance alone is rather interesting.
The Light is dimmer than he thought it would be. Sure, the Dark smothered much of the Light on the planet by sheer dint of Sidious and Vader being present, but even here in the heart of the Jedi Temple, what little Light remains is like a guttering plasma torch.
No matter. The fate of the Jedi Order is ultimately set in stone. While Vader doesn't intend to let Sidious win, he also doesn't expect the Jedi Order to accept his own plans for the galaxy going forward. The time for a Republic is over and done. There WILL be an Empire, no matter what. It just won't be Sidious' Empire.
Finally, Vader is able to locate Depa's Force Signature. The near-human Jedi Master is in the Training Halls right now, working her way through her forms with a lit blue lightsaber. That's good, because even if she's not directly calling upon the Force right now, she's still using it. Especially since she's practicing Vaapad.
Closing in on her, Vader 'watches' more closely, while carefully probing her Force Presence with his own. There's certainly something there, but the problem is synonymous with the reasons he sought her out first in the first place. Put simply, a Vaapad User looks almost exactly like a Dark Side User in the Force, at least on a surface level. He has to go deeper.
It's not too difficult to be fair. All he has to do is find an opening… and Depa's shields are actually filled with holes. As she dips closer to the Dark Side for another of Vaapad's katas, Vader dips in under her notice, slipping through with her none the wiser.
What he finds… is precisely what he was hoping for. She's Fallen. Truthfully, it's a little startling. Sure, he'd gamed it out and found the one that made the most sense to him logically, but that didn't mean Vader really thought he would find a Fallen Jedi hiding in plain sight on the Jedi High Council this easily.
Setting aside Shaak Ti because she was half a galaxy away from any other Jedi, this was… rather wild. That any Jedi, even a Jedi Master, could successfully hide from her fellow Councilmembers while sitting in the same chambers as them day after day… well, it truly spoke to how blinded the Jedi were in these final days of their order.
Still, Vader had his target. There was indeed a Fallen on the Jedi High Council and it was Depa Billaba. At least now he knew. The only question then became… what was he going to do about it?
He's barely begun turning the question over in his head before he's on his feet and leaving the Room of a Thousand Fountains behind. Making his way to the private training hall where Depa is currently working through her lightsaber katas alone, Vader can't quite help himself.
When he arrives outside of the door, he palms the door panel and then speaks into the intercom.
"Master Billaba. I thought you might like a sparring partner."
Obviously he doesn't have the authority to override a Jedi Master's codes and just enter the room without her permission. So he has to appeal to her current need for violence instead. Fortunately, that seems to work because after a long moment where she hesitates, the door does finally slide open, revealing her staring at him curiously.
"Knight Skywalker."
She doesn't say anything else, but then she also really doesn't have to. If she didn't want to take him up on his offer, she wouldn't have opened the door and let him in. Stepping into the room, Vader shucks off his robes and smiles slightly as he moves to join her in the center of the arena. He unclips his saber from his belt before offering her a perfunctory bow.
"Please Master. I am in your care."
"… Very well."
They both face off with one another, Vader's currently blue saber activating with a snap-hiss. For a moment, everything stands still and silence aside from the hum of their blades fills the air. Then they meet in the middle of the floor, their blades clashing.
As expected, Depa's Vaapad is brutal and aggressive and fast-paced. Unfortunately for her, Vader is one of the worst opponents for a Vaapad user to face.
Back when he'd just been Anakin Skywalker, his preference was Djem So blade strikes with Ataru Footwork, allowing him to turn the powerful strokes of Djem So into a tornado of death that could take him anywhere on a battlefield at any time.
However, the suit… the suit had weighed him down significantly. It was heavy and clunky and stripped away his ability to use Ataru Footwork altogether. In the end, Vader had wound up merging Juyo, Soresu, Shien, and Makashi into a single style, one that allowed him to take on any kind of opponent that might come at him, whether it was fugitive jedi, opportunistic bounty hunters, desperate rebels, or otherwise.
As Anakin, he was a tornado of death. As Vader, he'd had to become a wall of death.
But now he was back. Anakin Skywalker's body, Darth Vader's experience. Everything he was, everything he'd ever done, had turned him into a proper killing machine.
Nothing Depa could do with Vaapad could touch Vader. That much he knew immediately and instinctively. The question therefore became… did he want HER to know that? How exactly did he want to play this? He knew he wanted to figure out what made her Fall. He wanted to see why she had turned to the Dark Side.
But was utterly destroying her the best way to go about exposing such a secret? Or was there another path entirely that would ensure better results while keeping his true capabilities a secret? He could… toy with her. Instead of utterly dominating the fight, he could annoy her into revealing her true nature, maybe.
Hm… decisions, decisions.