WARNING: This article contains SPOILERS for the Darth Vader comic series
The origin story of Darth Vader's red lightsaber has finally been told. The creation of the Sith blade is explained in the pages of the newest Darth Vader comic series. It will come as news to fans who thought they knew the lightsaber's story - with the expanded universe wiped out, the challenge has largely fallen to the new batch of Star Wars comics from Marvel (and novels, TV shows, and films) to establish a new canon. Beginning with the story of Darth Vader, and how his final step in becoming a Sith Apprentice was actually his iconic weapon.
Just moments after Vader awakens in his helmeted, artificial form in Darth Vader #1, Emperor Palpatine asks his apprentice to produce a lightsaber. Not the one used by Anakin Skywalker - that's now held by Obi-wan Kenobi, still years away from being passed to Luke. The Emperor is referring to the lightsaber of Darth Vader. As the pair look out on Coruscant (and the public destruction of every Jedi lightsaber) Darth Vader realizes his first mission.
The lightsaber of a Sith must be TAKEN.
Darth Vader Kills a Lost Jedi For His Lightsaber
Obviously this requirement poses a bit of a problem, since the Sith Lords have just finished executing Order 66 and slaughtering every known Jedi across the galaxy. So in finding one to defeat in combat, and thereby claim their lightsaber as his own, Darth Vader has some digging to do. In the process, revealing yet another new detail added to the Jedi mythology: the Bashar Vow. A vow taken as penance by a Jedi, committing to disconnecting themselves from everything but The Force. They may observe and absorb all actions and conflict within the Jedi Order, but do nothing. Suspecting that such a Jedi outcast would have survived the Jedi Purge, Vader seeks one out (this is before he learns that the Jedi librarian Jocasta Nu survived the Purge as well).
Heading to a distant river moon, Vader finds one of the few Jedi left in the galaxy: a champion fighter named Kirak Infil'a. And once Kirak Infil'a realizes the truth - that this monstrous figure of the Dark Side has wiped out the Jedi Order - he invites him to open combat. The initial fight is short, with Kirak besting Vader's artificial limbs, tossing him to his death, and immediately planning to found a new Jedi Order.
But this is Darth Vader we're talking about. After reassembling his robotic body, Vader unleashes his rage not upon the Jedi, but the innocent civilians of a nearby city. Kirak Infil'a is distracted, defeated, and dispatched along with countless innocent lives. Bearing the lightsaber of Kirak Infil'a - the first canonical lightsaber of the reborn Darth Vader - the Apprentice receives his next challenge.
Darth Vader Returns To Mustafar
The kyber crystals of the Star Wars Universe have taken the spotlight under Disney's watch, becoming major plot points in Rogue One, Star Wars: Rebels, and more. But for longtime fans who know how Sith lightsaber got their red color in the old, Legends continuity... things have changed. Previously, the Star Wars canon explained that only Jedi use the naturally formed kyber crystals to power their lightsabers. And while red crystals do occur in nature, the fact that Jedi used and controlled the biggest kyber crystal sites means they are off-limits to the Sith.
Instead, they create synthetic crystals - another example of the Jedi submitting to the mysteries of the universe, while the Sith exercise control. When they are forged, Sith make their crystals red as a tribute to the "bloodshine blades" of their ancient ancestors.
But that was the OLD story. The new canon gives a new reason. A much, much darker reason...