The Death Stars were moon-sized battle stations developed by the Galactic Empire, capable of destroying entire planets. Thirty years later, the First Order followed suit, tunneling the kyber-rich planet Illum to build the Starkiller Base. That's par for the course for a militarized regime, but it always seemed like a waste of resources, given the Emperor’s objectives and his explicit disregard for building lasting political structures. But what if those feats of wartime engineering weren't what the Imperial and First Order officials thought they were? What if the true reason for their existence was to allow Palpatine to achieve immortality? That’s what the VR game Vader Immortal seems to be pointing at, through the legend of Lady Corvax and the Brightstar.
In Vader Immortal, the player character ventures beneath Fortress Vader on Mustafar to retrieve the Brightstar artifact for Darth Vader. Lady Corvax’s spirit guides the player on this quest, and along the way she explains that she stole the Brightstar, the sacred object that provided Mustafar with life, to create a device that would bring her dead husband back to life. (That's why Vader wants the Brightstar too: He’s trying to bring Padmé back to life.)
However, while a portal was created, and her husband was trapped in the world between the living and the dead, the Brightstar energy destroyed Mustafar, transforming it into the volcanic inferno that it is now. The portal remained active, a rip in time and space, and it was a major plot point the Marvel comics series Darth Vader: Dark Lord of the Sith, in which Vader is also on a quest to resurrect Padmé (he blunders, although not because the portal wasn’t working).
Vader Immortal peppers the screen with visual hints that this Brightstar is actually primitive Death Star technology: The Brightstar looks like a massive, spiral kyber crystal, within a cave of smaller kyber crystals. The pre-Corvax Mustafarians had been using it as an energy source, or perhaps as a way to keep their world in balance in the Force. Lady Corvax broke that balance to rip the veil of reality and bring back her husband, which caused the devastation.
Canon material backs up that hypothesis: In Catalyst: A Rogue One novel, scientist Galen Erso discovers that kyber crystals could be faceted to become a very cheap source of energy for entire planets. He also proposed that kyber crystals were able to align the natural “dark” and “light” side of the Force to focus, balance and increase this energy output even more. However, without proper containment this energy would lash out and destroy everything that surrounded it – as it could be seen in the Death Star I, II and the Starkiller Base, and on a lesser degree, Kylo Ren’s fractured kyber crystal and crackling, spitting sword, and in the explosion that results from Anakin's lightsaber breaking in The Last Jedi. That's exactly what happened to Mustafar after Lady Corvax unwittingly unleashed the energy of the Brightstar.
The Emperor was obsessed with immortality, and everything he did was designed to lead to that goal. Why bother to harness such a vast and time-consuming military power and building two massive and incredibly conspicuous space stations “just” to kill people? Could he have been trying to use the same kyber crystal energy around which the Death Stars were based to open a portal to a higher plane of existence, beyond life and death?
There is a plane like that: In The Clone Wars, the Gods of Mortis seem to inhabit and control a space where anything can happen. On Star Wars Rebels, Palpatine uses a Mortis portal to penetrate the World Between Worlds, the place in which the Force connects all points in time and space; staying there would have made him invincible. In the comic book Darth Vader: Dark Lord of the Sith, Palpatine implies he knows about the portal under Mustafar, so he probably knew the legend of the Brightstar and its resurrection properties too. All of that, plus the Rur crystal (more on that in a moment), point toward the idea of an advanced civilization that knew how to wield the kyber crystals better than the old Sith and the Jedi. They went extinct, leaving behind only legends, artifacts and gods: a tempting puzzle for a Sith so obsessed with power and eternal life.
The Force Awakens featured Starkiller Base, which was originally the kyber-rich planet of Illum, the place where Jedi padawans went to retrieve crystals for their lightsabers. The First Order took advantage of the extinction of the Jedi to take over the planet, tunnel it and exploit its core to create the most terrifying weapon of mass destruction: one that killed suns and destroyed entire systems across impossible distances. In the novelization, it is mentioned they did this by ripping a hole through time and space. In Finn’s words, “It doesn't operate in what we'd call normal hyperspace. It fires through a hole in the continuum that it makes itself. Everybody was calling it 'sub'-hyperspace." In a different internal monologue, General Hux, closest thing the First Order has to a chief science officer, thinks about how Starkiller melds the “quintessence” derived from dark matter with the kyber crystals at the core of the planet to create “phantasm energy” that can travel through the galaxy instead of across it.
It sounds as if the First Order unwittingly created a rip in space and time that shortcut through the World Between Worlds. Within The Force Awakens, there are some interesting Force-related events that occur at the same time as the Starkiller weapon is being charged and deployed: Rey and Kylo Ren (two Force users) are on the base as the Starkiller, a massive machine surrounding an enormous amount of kyber crystals, sucks the sun dry. Kylo Ren surrenders to the Dark Side of the Force and kills his father. Poe Dameron’s targets the oscillator that ultimately destroys the planet, which coincides with the turning of the tables in the fight between Kylo and Rey. In the novel, interestingly, that's also the precise moment when Rey hears a cold, murderous voice in her head coaxing her to kill Kylo. It's a voice that sounds a lot like the Emperor talking through time and space. Why at that precise moment, and not before, unless Palptine's ability to communicate with the living was dependent on the existence of this hole and the presence of a kyber crystal planet?
But what about resurrection? Other than by ripping the fabric of reality and the balance of dark and light, is it possible for a kyber crystal to help someone come back to life, like Lady Corvax intended? We know that it’s possible for kyber crystals to hold conscience and Force powers, even for Jedi. In Marvel's canonical Doctor Aphra, the characters finds the ruins of the space-station fortress of the Ordu Aspectu, an ancient, pre-Republic sect of Jedi that wanted to achieve immortality. Their Rur crystal (another artifact that looks an awful lot like a massive kyber, but this time in green) was the computer core of their fortress, its source of energy and the vessel for the Eternal Rur, a corrupted Jedi spirit A.I.
Darth Vader eventually fetched the Rur Crystal, and it became a prized part of Palpatine’s collection. So, yes, in a way, kyber crystals could technically be used to contain the conscience of Force users. This ancient galactic concept, combined with the legend of Lady Corvax, would work perfectly for Palpatine’s quest for ultimate power and immortality, and would explain his very disparate interests in ancient history and technological advancement: all along, he was trying to replicate and improve upon these ancient technologies for his own benefit and his ultimate goal: immortality.
When Darth Vader killed the Emperor, he did so by throwing him deeper into the heart of the Death Star, where he exploded into energy. Part of that energy was seen hitting Vader’s helmet (that might explain why it speaks to Kylo Ren), but most of it went down the chute, toward the kybers powering the Death Star's core. We know that Sith can attach themselves to objects, so what object would be better for Palpatine’s spirit than the core of a Death Star, made up of kyber crystals that were probably being tuned to manipulate reality itself?
If Palpatine’s spirit is indeed stuck in the Death Star, that would explain why Rey, Poe, Finn and probably Kylo Ren are heading to it in The Rise of Skywalker: they want to reach him before the other side does, and prevent him (or assist him) in escaping before another kyber-crystal is used to destroy. And, judging by the red-rimmed Sith Fleet with miniaturized Death Star cannons that blow up Kijimi in the Episode IX trailer, it seems like the amount of tiny starkillers able to assist him has multiplied a thousand-fold.