In the movie, all we really had to explain Palpatine’s comeback was that the Dark Side is a “pathway to many abilities some would consider unnatural”.
Thankfully, Star Wars: The Rise of Skywalker (Expanded Edition), written by Rae Carson, has revealed just how the evil Sith Lord returned, confirming a common fan theory about his resurrection.
During Kylo Ren’s first meeting with Palpatine, he decides to take a close look at the machinery connected to the resurrected Emperor.
“He’d seen this apparatus before, too, when he’d studied the Clone Wars as a boy. The liquid flowing into the living nightmare before him was fighting a losing battle to sustain the Emperor’s putrid flesh,” the novel reads (via ScreenRant).
And then, as predicted, the novel confirms that the Palpatine we’re seeing is a clone:
“Kylo could feel in his very bones that this clone body sheltered the Emperor’s actual spirit. It was an imperfect vessel, though, unable to contain his immense power. It couldn’t last much longer.”
In the Expanded Universe, the Dark Empire comics by Tom Veitch and Cam Kennedy revealed that Palpatine had the ability to transfer his consciousness between cloned bodies.
While those comics are no longer considered canon, that idea is now back in Star Wars canon as the novelisation confirms this is essentially what Palpatine has done.
As in that series, apparently Palpatine had all these clones ready for his potential demise, so that after his death in Return of the Jedi, his spirit could return in one of them so that he could carry out his Final Order plan.
The difference in The Rise of Skywalker is that, unlike in the Dark Empire comics, his clones couldn’t quite contain his power.
Having failed to get Rey to kill him, his failing clone body would be why Palpatine decided instead to sap the Force dyad energy from Rey and Ben Solo, giving him enough energy to revive himself to his full evil glory.
Sure, it didn’t last long, but who’s to say Palpatine didn’t have clones elsewhere in the galaxy, and could return in future movies?