The Death Star shows how powerful Sith lightsabers really could be in Star Wars. A lightsaber is far more than a mere weapon; the kyber contained within it means a lightsaber has been described as "a lens that focuses the light inside the Jedi." The Sith commit sacrilege by twisting kyber crystals to the dark side, a process called "bleeding" because it turns the crystal red. But the true power of Sith kyber crystals is demonstrated by the Death Star.
The Death Star was powered by vast amounts of kyber, after the scientist Galen Erso figured out how to unlock their power. Erso's process is described in James Luceno's novel Catalyst, and he actually seems to have discovered a way to "bleed" kyber crystals using science, because the process turned them a familiar blood-red. This suggests the Sith, too, have turned the kyber crystals into a scar upon the Force, a reservoir of dark side power that they can potentially tap into and unleash.
Kylo Ren Came Close To Tapping Into The Power Of Kyber Crystals
It's likely the ancient Sith knew this; as Yoda reflected in Star Wars: The Clone Wars, they possessed countless devastating superweapons powered by kyber. The secret must have been lost in the mists of time, though, because it was rediscovered by Galen Erso in the Dark Times of the Empire's reign. Oddly enough, only Kylo Ren came close to discovering this kyber power; he used a crystal that had cracked as he bled it, and according to the Star Wars: Force & Destiny sourcebook Unlimited Power, the Force can only be drawn from such flawed crystals. Kylo Ren knew many secrets of the Force even Palpatine did not, realizing he was part of a Force Dyad before the Emperor, but there's no evidence he ever learned the true potential of his lightsaber.
What Could A SIth Potentially Do With Their Lightsaber's Kyber Crystal?
This naturally raises the question of just what a Sith could do with their lightsaber if they tapped into the power of the kyber crystal contained within it. According to the sourcebook, a Sith skilled in particular dark arts can perform a lengthy ritual that turns the crystal's power into a reservoir of energy, boosting their own natural Force abilities. As powerful as Darth Vader may have been, for example, even the Chosen One would have been happy to give himself this kind of power-up. He'd probably have used this to manifest even greater feats in the Force, perhaps boosting his precognitive awareness of a coming attack, or further strengthening his telekinetic grip. Palpatine, who didn't tend to use his lightsaber particularly often, would perhaps have considered the crystal a useful tool had he known the right ritual.
The sourcebook is silent about whether a kyber crystal can be repeatedly used in this manner, though. The Death Star required vast amounts of kyber to power its superlaser, but it's entirely possible repeated shots would have drained the reactor of power over time. In the same way, a Sith who used kyber to boost his own dark side Force powers may well have used it up, meaning they would then need a replacement. If this is the case, it may well explain why the Sith forgot (or abandoned) this technique by the time of the main Star Wars saga.