It was previously revealed why Sith lightsabers are red. It's because the Kyber crystals inside of them are alive and they bleed. But how does a Sith go about constructing their lightsaber? That is a myth that is finally revealed and made part of the official Star Wars canon in a new Star Wars comic book. And it's just as ominous and evil as any fan can imagine. Basically, the Sith torture the living crystal, filling them with immense pain.
As any Star Wars fan worth their weight in action figures can attest, a Sith's lightsaber is always red. And that color comes from the Kyber crystal inside. While George Lucas didn't have any substantial backstory for this when he made A New Hope back in 1977, Disney is fully fleshing out this universe, and giving every minuscule thing a reason for existing. Where Lucas just thought it was cool for bad guys to have red lightsabers and good guys to have blue, it turns out there is a more substantial reason than just the aesthetics of visual filmmaking as an art form. And the reason feeds into the story.
Marvel has released a new comic called Darth Vader #1, which fills in the Sith Lord's backstory between Revenge of the Sith and Rogue One. Fans finally get to see Anakin's early days in the Darth Vader suit, and how he struggled with accepting his fate. We also get to see Darth construct his now iconic red lightsaber with a little help from his mentor Emperor Palpatine.
For the first time we get to see Darth Vader and Palpatine training together. In one very interesting scene, the curious pupil asks his master about the red lightsabers, having lost his blue lightsaber during his fight with Obi-Wan Kenobi on his future home of Mustafar. Mostly he wants to know how they get their color.
As explained in a previous Star Wars book, Palpatine confirmed that the crystals are crying blood. These Kyber crystals used in Sith lightsabers have been stolen from the Jedi, and they once were regular Kyber crystals. The crystals live in their own unique way, and they do feel pain. When a Sith takes one of the crystals away from a Jedi, and use the dark side of the force, they pour their rage and anger directly into the crystal, causing it immense pain in the process. Palpatine explains it to Darth like this.
"The crystal from any Jedi's saber will do. As you know, the Kyber's are alive, in their way. Like any living thing, they can fell pain. Through the dark side, you must pour your pain into the crystal. And when, at last, the agony becomes more than it can stand...a beautiful crimson. The color of your rage."
You can see the page from the comic book which fleshes out this scene in full. The comic book will further show how Darth constructed his lightsaber, which is believed to be the one used all throughout the original trilogy. Some believed that Luke Skywalker is wearing the Kyber crystal once belonging to his father on a necklace in The Last Jedi, but it was later officially confirmed by Lucasfilm that Darth's old lightsaber was completely destroyed along with the second Death Star in Return of the Jedi. Check out the comic book page thanks to ComicBook.com