When Emperor Palpatine called for the creation of new apprentices to challenge Darth Vader, the Sith Lord brutally took on the challengers and killed them. In an effort to make something better than Vader and gain progress for the Empire, Palpatine had Doctor Cylo create and bring together a number of potential new apprentices. Unfortunately, those apprentices stood little chance against the wrath of Darth Vader.
Emperor Palpatine routinely tested Darth Vader on his way to becoming his apprentice and leader of the Sith - including dismantling his greatest student and forcing him to put himself back together. One of Vader's biggest tests came when he was desperate to prove himself to his master. Palpatine worked alongside Doctor Cylo, an advanced technology specialist, to create a number of cyborgs that were made to disrupt Vader's place as his true apprentice. While Cylo believed his creations would be capable of taking down Darth Vader once and for all, they ultimately served as fodder for the Sith to show his worth to Palpatine.
In Kieron Gillen, Salvador Larroca, and Edgar Delgado's Marvel Comics' Darth Vader series, Vader learns Palpatine has brought together replacements for him while seeking out Doctor Cylo's base of operations. When he arrives, he immediately battles two lightsaber-wielding opponents named Morit and Aoilin, but a new version of Cylo (Cylo-V) appears and calls for them to stop. When Vader asks him if he was making new apprentices, Cylo-V tells him he had actually constructed them under the request of Palpatine.
The replacements include the two lightsaber-wielding siblings, a Trandoshan warrior, a scientist, and the rebuilt Mon Calamarian commander, Karbin. A battle to the death commences, with Aoilin killing the Trandoshan warrior before he gets to Vader. Palpatine stops the fight and tells Vader he disappointed him on Mustafar and will have to prove himself worthy again. Vader's anger would lead to the deaths of the apprentices who were supposed to replace him.
Throughout the rest of the series, Vader would pick off the apprentices one by one. While Aiolin is killed after her brother pushes her into lava after she says they can't win against Vader, Morit's death is equally as brutal. Vader crushes him with metal beams on Cylo's ship, as he's ironically burned alive after being sucked into the atmosphere of a nearby planet. Against the enhanced scientist Tulon Voidgazer, Vader takes control of one of her droids and makes it shoot and kill her.
The replacement who got it the worst from Vader is Karbin. The Darth Grevious-esque enhanced warrior dueled the Sith Lord with four lightsabers, but thanks to an assist from Doctor Aphra (who drove her ship into his opponent), Vader was able to bring Karbin's mangled body back to Palpatine to prove his win. Vader proved in the series that nobody was capable of replacing him, no matter how many technological advancements and weapons they were given. Darth Vader's replacements created for Emperor Palpatine ultimately stood no chance against the Star Wars villain as their defeats remain among his most impressive victories to date.