Palpatine returned in the Dark Horse comic book series Dark Empire, written by Tom Veitch and illustrated by Cam Kennedy in 1991.
In the series (set six years after Return of the Jedi), Palpatine is resurrected as the Emperor Reborn or “Palpatine the Undying.”
His spirit returns from the netherworld of the Force and possesses the body of Jeng Droga, one of the Emperor’s Hands, a group of elite assassins not unlike the Knights of Ren or Snoke’s Praetorian Guard in the Sequel Trilogy.
“He feels that whatever the outcome of this confrontation with Luke, he, Palpatine, will conquer.”
According to Veitch, the Emperor chose this moment to come out of the shadows because he no longer feared for the safety of his physical form. With one swift stroke, Luke would fall to the dark side, and Palpatine would be reborn.
“His mastery of the dark side had become such that he was now ready to make a transition he had been working toward for many years — namely the replacement of his aging, diseased, and crippled body with a young clone.”
Operation Cinder
Palpatine had a plan, fitting in with what we know of Force ghosts in the Star Wars canon and some visual hints we’ve seen in Battlefront II comics and video games.
Palpatine recorded his visage into various droids so he could make orders after his death and destabilize the Empire and the Republic.
Palpatine's contingency plan upon his death included sending numerous droids out across the galaxy to give his final orders for what he called “Operation: Cinder,” a method of destabilizing the Empire and the Republic after his death so that no person would ever have the amount of power he did.
These red-cloaked, humanoid figures resembled the Red Guard but used a projection of Palpatine’s face on their own heads.
Possessing a Knight of Ren is a different thing entirely, but the visual of Palpatine existing after death by forcing his identity onto another being is Star Wars canon.
