The Inquisitorius was a program founded by Sheev Palpatine shortly after he proclaimed himself Emperor, and he soon placed it under the command of Darth Vader.
The Inquisitorius was made up of Force sensitive beings, former Jedi, who were trained and tasked with hunting down the remaining Jedi who survived the Purge. Vader cruelly trained them to face Jedi, but since they were not themselves Sith, they were not as adept at dealing with major threats as Lord Vader. Still, the Inquisitors were largely successful in their task, as they helped carry out the eradication of many Jedi, leaving very few left in the galaxy by the time of the Battle of Yavin. The Inquisitors served the command of Darth Vader and themselves commanded Purge Troopers.
The Inquisitors were based at Fortress Inquisitorius, located on the planet Nur in the Mustafar system. In order to keep the existence of the Inquistors largely a secret, the location of this remote hideout was kept hidden from much of the galaxy. It served as a prison and torture facility for captured Jedi, including Cere Junda.
Here’s a list of all the Inquisitors we know about (so far), and beware: ***significant spoilers for Jedi: Fallen Order are ahead!***
The Grand Inquisitor was a former Jedi Temple Guard who held the highest rank among all Inquisitors. Under the command of Darth Vader, the Grand Inquisitor out-ranked all other Inquisitors and Purge Troopers and was thus more well-equipped and well-trained than the others to hunt and kill Jedi. He was sent to find and kill Jedi Kanan Jarrus and his apprentice Ezra Bridger, and he succeeded in capturing Jarrus and bringing him back to Mustafar for torture. But when the Ghost crew arrived to rescue Jarrus, the Grand Inquisitor dueled the Jedi and was defeated. He then chose to fall to his death rather than face Darth Vader as a failure, something he called “far more frightening than death.”
The Second Sister was a Jedi formerly known as Trilla Suduri, who was the apprentice to Cere Junda. Master and apprentice survived Order 66 and took with them a group of younglings to hide. Junda was captured, however, and upon much torture gave up the location of her padawan. Suduri was also captured and tortured, and eventually she broke and embraced the dark side, becoming an inquisitor. At some point, she carried out a mission to Ontotho. Later, she and the Ninth Sister journeyed to Bracca to investigate possible Jedi activity, discovering Cal Kestis. The Second Sister fought Kestsis on Bracca, but the young Jedi was rescued by Junda and her pilot, Greez Dritus. She later encountered Kestis again on Zeffo, revealing that she knew of his search for the holocron and that she wanted it too. Her intentions were apparently to earn the Emperor’s favor by acquiring the holocron that gave the location of Force sensitive children. Kestis escaped this confrontation thanks to his droid, BD-1, but the Second Sister again encountered him on Bogano, retrieving the holocron and taking it back to Fortress Inquisitorius. Kestis and Junda infiltrated the Fortress to retrieve the holocron and Kestis this time overpowered the Second Sister. Junda offered Suduri her apology, but Darth Vader arrived and killed the Second Sister for her failure. Right before she died by Vader’s blade, she pleaded with Kestis and Junda to “avenge us”.
The Fourth Sister was a former Jedi, but not much is known about her. We mainly know about her because Lina Graf, a Rebel commander, stole the Fourth Sister’s armor and tried to impersonate her to infiltrate Vader’s castle on Mustafar. We actually know nothing about the Fourth Sister herself, only that she was the one who was impersonated by Graf.
A former Jedi, the Fifth Brother continued training under Darth Vader and the Grand Inquisitor. The Fifth Brother was a part of Vader’s strike team in pursuit of former Jedi High Council member Eeth Koth, and many years later was assigned to hunt down Jedi Kanan Jarrus and his padawan Ezra Bridger. Along the way – and as he continued with the Empire’s mission of abducting Force sensitive children – he, along with the Seventh Sister, encountered Ahsoka Tano. Later, he encountered Jarrus, Bridger, Tano, and Maul on Malachor, where he was killed by Maul. Though he was physically imposing, he was not much of a match for skilled lightsaber wielders like Tano or Maul.
The Sixth Brother was a former Jedi known as Bil Valen. Upon joining the Inquisitorius, he lost his arm in training with Vader, and later was part of Vader’s mission to Mon Cala. In order to help ensure his own escape from their predicament on Mon Cala, the Sixth Brother betrayed the Ninth Sister and left her behind, further proving that he had no problem harming or killing others for self-gain or self-preservation. At some point after that, he was hunting a Force sensitive child when he got word of an older Force user on Raada, who turned out to be Ahsoka Tano, and hunted her down before he was killed by Tano.
The Seventh Sister was a former Jedi who was sent out to hunt for Ahsoka Tano during the few years leading up to the Battle of Yavin. This mission brought her into contact with Kanan Jarrus and Ezra Bridger as well, and she worked closely with the Fifth Brother to continue hunting the Jedi down – including taking Force sensitive young children. She soon found Tano but was defeated in combat, with the former Jedi padawan escaping. The Seventh Sister met up with the Jedi again on Malachor, facing off against Jarrus, Bridger, Tano, and Maul, where she was subsequently killed by the former Sith Maul.
A former Jedi, the Eighth Brother was dispatched to hunt down Maul, known as “the shadow.” He tracked him to Malachor, where the Eighth Brother joined forces with the Fifth Brother and Seventh Sister to face Maul, along with Kanan Jarrus, Ezra Bridger, and Ahsoka Tano. The Eighth Brother soon attempted to retreat from this fight by using the helicopter-like nature of his spinning lightsaber, but his weapon had been damaged and he fell to his death.
Formerly a Jedi known as Masana Tide, the Ninth Sister was captured and tortured before she became an Inquisitor. She was part of Vader’s team investigating possible Jedi activity first on Cabarria, and then on Mon Cala. While on Mon Cala, the Ninth Sister was betrayed by the Sixth Brother, who cut off her leg and left her behind in order to escape himself. But the Ninth Sister survived this and years later went with the Second Sister to Bracca to again investigate possible Jedi activity, which turned out to be Cal Kestis. Kestis escaped from their grasp on Bracca, but the Ninth Sister tracked him down on Kashyyyk, eventually confronting him atop the Origin Tree. After a lengthy duel, Kestis eventually got the upper hand and used the Force to push the Ninth Sister off the giant tree, sending her falling to her apparent death.
The Tenth Brother was formerly a Jedi known as Prosset Dibs, a former Jedi Master who grew disillusioned with the ways of the Jedi and, during a mission with Mace Windu, committed treason by trying to kill Windu because of it. He was spared death because of Windu’s pleading to the Jedi High Council, but once the Republic fell Dibs joined the Inquisitorius. He was part of Darth Vader’s team to Mon Cala in search of a Jedi, but when they encountered the Jedi, he persuaded the Purge Troopers (who were former clones) to turn on these former Jedi by executing Order 66. The Tenth Brother was killed soon after the Purge Troopers opened fire.
There are other Inquisitors that have been pictured, mentioned, or described in various media as well, but they have not been given names yet. So there are others besides the ones listed here, but these are the ones we know of so far.