The Star Wars universe is vast and always revealing something new to learn. However, before the new canon was set in place, the Legends canon featured an even larger world with nearly double the characters and wildly different perspectives on famous events.
Some of the most action-packed stories could be found in the old Marvel Comics Star Wars series. One tale in particular revolved around the inclusion of a secret apprentice that Darth Vader taught. His name was Flint, and he had a similar backstory to Luke. Both of their fathers were Jedi, and he too was Force-sensitive. When Flint first met the hero, Luke brushed him away with no desire to teach him. He still aided the Rebels in stopping the Empire, but it came at the cost of losing his mother. Grief-stricken, he was found by Darth Vader, who promised to teach him the ways of the Dark Side.
Most Sith, Vader especially, have a history of taking on a secret apprentice behind their master's back. Vader had also taken up another apprentice named Lumiya, and he hoped that whichever would win in combat would be his shadow hand when they decided to overthrow the Emperor. This would not come to pass, as Vader died before his treachery could be seen to the end. Flint saw Vader as a surrogate father, and his death pushed him farther into the Dark Side and on a collision course with Luke Skywalker.
However, before he could take on his enemy, he first had to battle Lumiya for leadership. Ultimately, he lost, but he was still tasked with taking on the Jedi, which he gladly agreed to. Luke, on the other hand, was warned by the spirit of his father that only he could save his old apprentice. The two took part in a deadly duel. What turned the tide was an old friend of Flint's who was assumed to be dead. This friend pleaded with him to leave the Empire, and after some hesitation, the Sith renounced his dark ways.
Flint went on to serve time for his misdeeds and live on his home planet. While there were still problems that befell the planet, he lived out the rest of his days in relative peace. But that didn't last long as the powers of the Dark Side wouldn't go away that easily. Years later, his old master, Lumiya, found Flint and killed him. His body was found with a stab wound through the throat, possibly by a lightsaber.
When Flint first appeared in the Star Wars comic series, he was considered an evil Luke Skywalker. Everything about his story mirrored a possible future for the young hero. Because Flint and Vader had a father-son relationship, Luke and the dark apprentice were like siblings who were forced on two separate paths. Luke would always feel some regret for how he treated Flint when he was younger. But thankfully, Flint managed to turn his life around and do some good before his untimely end.
Years ago, it would have seemed impossible for a character like Flint to appear in the proper Star Wars canon. But with so many shows coming out, it feels like anything is possible. Characters like Ahsoka and Bo-Katan have already appeared, and lightsaber duels have also been shown, so if there is any chance for Flint to appear, now would be the best time. Who knows, maybe he will be an enemy for a future season of The Mandalorian.