There's nothing that says "Sith" more than a shimmering crimson blade. From the moment we first saw Darth Vader wield his lightsaber in A New Hope, fans have been fascinated by it. Where Jedi use blades of varying color — usually green and blues, although Samuel L. Jackson argued for a purple one — we typically see dark siders, and especially Sith, carry lightsabers with a fierce crimson hue.
Where Do Sith Get Their Lightsaber Crystals From?
At the heart of every lightsaber is a Kyber Crystal. These are living crystals, attuned to the Force, and they contain great energy. In fact, the Death Star's phenomenal superlaser was powered by Kyber Crystals! Jedi traditionally seek out a Kyber Crystal, and integrate it with their lightsaber. But the Sith do something very, very different.
Charles Soule's Darth Vader miniseries is set straight after the events of Revenge of the Sith, and it begins with Palpatine sending his new Apprentice on a quest — and on something of a test, too. Vader lost his old lightsaber on Mustafar, but Palpatine tells him that doesn't really matter. He'll need to forge a new lightsaber anyway; and to do that, he'll need a new crystal.
Disturbingly, Palpatine explains that a Sith's crystal isn't found, or given. It is taken. Over the millennia, every Lord of the Sith has tasked their Apprentice to seek out a Jedi and kill them, taking the Jedi's lightsaber as their own. It's a bold strategy — especially when you remember that, for millennia, the Sith have been in hiding. If Palpatine is telling the truth, then the Sith have always risked discovery; every new Sith Apprentice has been commanded to brave a battle with a Jedi. What's more, they did this before they even carried a lightsaber, proving themselves worthy to become a Sith.
This revelation adds a whole new element to the Prequel Trilogy. Remember Darth Maul? He wielded a double-bladed lightsaber, with two crystals in it. That suggests that, back when Maul was tasked to create his lightsaber, he didn't just kill one Jedi. He killed two. Not only does this make Maul's weapon all the more impressive, it also dovetails nicely with events in Marvel's Darth Maul miniseries. Set before The Phantom Menace, that miniseries showed Maul struggling to adhere to the Sith's commands to be cautious; he was desperate to slaughter more Jedi. It's no wonder he was frustrated; Maul started off by killing two Jedi, only to be forced back into the shadows by Sidious.
Bleeding The Crystal
The novel Aftermath: Empire's End explains how a Jedi bonds with a Kyber Crystal.
"Just as the Jedi are a lens that focuses the Force, so is the kyber crystal a lens that focuses the light inside the Jedi - and the light inside the Jedi's weapon, the lightsaber."
For a Sith, the experience is very different. A Sith uses the Kyber as a focus of their anger, their hatred, and their pain. A Sith bends the Kyber Crystal to the dark side, in a process known as "bleeding." We've heard mention of this many times before, but Darth Vader #1 is the most overt reference. What's more, given the book's plot focuses on Vader's quest to construct his lightsaber, it's possible the comic will actually show us Vader turning a Kyber Crystal to the dark side.
There's another subtle side to this process, though. In the Rogue One prelude novel, Catalyst, Galen Erso used science to bleed the Kyber. Disturbingly, doing so (even using artificial means) had a profound effect upon Galen. As the Kyber bent to his will, he began to grow more introverted, angry and obsessive; his mind clouded over with suspicion and bitterness. It's as though the experience of bleeding the Kyber actually amplified all that was bad in Galen's heart; now imagine what the experience must be like for someone who is Force-sensitive, and who bleeds the Kyber by direct force of will.
It's clear that, for the Sith, this is something of a point of no return; if an Apprentice's will proves strong enough to bleed the Kyber, the experience further exposes them to the dark side.
At last, the secrets of those shimmering crimson blades are finally being revealed. Each Sith lightsaber we see was taken from the corpse of a Jedi. What's more, every one of these lightsabers is a testimony to a Sith's force of will — a will that was great enough to bleed a Kyber, as a crucial step on the journey to becoming the next Sith Lord.