If you are a Star Wars fan who does not keep up with the ongoing Darth Vader by Marvel Comics, you probably have no idea what happened with the Sith Lord between The Empire Strikes Back and Return of the Jedi. Turns out a lot.
In the original trilogy, we simply see Vader back to being Darth Sidious' faithful apprentice in Episode VI after trying to hatch a plan with his son, Luke Skywalker, to overthrow Palpatine and rule the galaxy together. Emperor Palpatine doesn't seem to know or mind Vader's actions in the movies, but that is not the case with the comics.
Reported by Screen Rant, it was revealed in Darth Vader #9, written by Greg Pak and illustrated by Raffaele Ienco, that Emperor Palpatine left his apprentice in the lava pits of Mustafar after the events of The Empire Strikes Back. He broke Vader's armor and prosthetic limbs as punishment for his transgressions, almost similar to how Obi-Wan Kenobi left him amputated at the end of Revenge of the Sith, only for Palpatine to find him and turn into the man-machine he later became.
To survive, Vader was forced to brush up on his old skills in machine parts and mechanics, which he had left behind in Tatooine all those years ago. He used spare parts from outdated and destroyed battle droids to rebuild his armor, all the while running away from the Sith assassin, Ocho of Bestoon. He also uses the parts to repair an abandoned starfighter from the Clone Wars era to escape the planet.
In this issue, Vader thoroughly proved that his armor was merely a means to keep him alive and breathing. His skill with the Force still made him the formidable Sith Lord feared across the galaxy. However, it seems Vader did feel like upgrading his already 30-year-old suit, which was further dilapidated with the patchwork addition of droid parts.
In the recently concluded Star Wars: Dark Droids comic event, Darth Vader made customized upgrades to his suit, allowing him to control droids, even cyborgs, remotely. He managed the feat after learning to create a closed-circuit hivemind system to puppeteer said cybernetics after the encounter with the villain of the event, Scourge.
This comic series serves as an excellent bridge between the events of the original trilogy. There is still around a year's time left from this point in the story until Vader delivers his son to Palpatine. While Vader seems on the verge of creating an army of mind-controlled droids, fans fear some terrifying fate to befall the former Jedi that would force him to show the immense loyalty to Palpatine that we see in Episode VI.