Warning: This article contains SPOILERS for Star Wars: The Rise of Skywalker.
Star Wars: The Rise of Skywalker brought Kylo Ren's journey full circle, with him actually making good on his vow to finish what Darth Vader started - albeit not in the way he intended. Played by Adam Driver, Kylo Ren made his debut in Star Wars: The Force Awakens. Despite being the son of Han Solo and Leia Organa, Ben Solo had already succumbed to the dark side prior to the start of the film. Using the adopted name of Kylo Ren, he was eager to match the legacy Darth Vader had built before he redeemed himself. A memorable scene from The Force Awakens even saw Kylo talking to the remains of Darth Vader's helmet, urging his grandfather to again show him the power of the dark side...so that Kylo may fully complete the mission Vader had failed to see through to the end.
At the time, that mission, in Kylo's eyes, was to eliminate Luke Skywalker and thus wipe the Jedi officially from the galaxy. As well as destroy the resistance and ensure that the First Order could rule unopposed. Those plans experienced a wrinkle in the now heavily retconned Star Wars: The Last Jedi, however. Developing a bond with Rey, Kylo found himself pulled to the light more than ever. The bond between the pair was further explored and deepened in Star Wars: The Rise of Skywalker. Though Rey spent a majority of the film running from and fighting Kylo, she eventually revealed that she had wanted to take his hand and join him when he'd previously asked her to. Equally, after mortally wounding him in combat, Rey chose to Force heal Kylo. The act had a number of added effects. As well as healing his stab wound, it fixed his facial scar and seemed to repair whatever psychological damage once drove him to the dark side.
Until that point, despite learning that everything had been a manipulation orchestrated by a genuinely creepy Emperor Palpatine, Kylo had remained steadfast in his original course. After being healed by Rey and a conversation with a manifestation of Han Solo, though, he official shed the Kylo Ren persona and returned to being Ben Solo. In his newly redeemed guise, he returned to Exegol to help Rey against Palpatine. Unfortunately, after dispatching the Knights of Ren, Ben had some of his lifeforce drained from him and was propelled into a ravine. Though he survived and managed to make the climb back to the surface, he emerged to find not only Palpatine destroyed but Rey also dead. Repaying Rey's earlier kindness, however, Ben used the last of his lifeforce to heal Rey and bring her back to life. As such, he achieved his aforementioned vow - albeit in an unexpected way.
After all, while George Lucas' original Star Wars trilogy largely saw Darth Vader as a villain, his final act painted him as more of a tragic figure. That tragedy was further fleshed out with Lucas' Star Wars prequel trilogy - which followed Anakin Skywalker's journey from a good-natured child, to a conflicted padawan, and ultimately to his painful rebirth as Darth Vader. Anakin's descent, however, never resulted from a simple lust for power. No, it was driven by Vader's desire to save Padmé Amidala's life, having experienced visions of her death. Unfortunately, that desire was used as a tool for manipulation by Palpatine and ended up being what eventually helped to bring it about.
Though Ben and Rey never got the chance to fully explore a relationship, their bond was undeniably a strong one. Many of Ben's actions were similarly factored around Rey - right until the very end. And Rey's kiss only emphasized that it was romantic in nature. So, while he may not have meant it that way when he made the vow, by escaping Palpatine's control and succeeding in saving Rey's life, Ben Solo still did indeed complete what Anakin started. As such, despite the divided reaction experienced by the film since its release, Star Wars: The Rise of Skywalker at least somewhat succeeded in bringing not only Ben Solo's journey within the new trilogy but an element of the Skywalker saga itself full circle.