Warning: spoilers for Star Wars: Darth Vader #11 and Star Wars: The Rise of Kylo Ren below.
Emperor Palpatine feared Luke Skywalker after the events of Star Wars: Return of the Jedi, admitting that with the Jedi by his side, he could have ruled the galaxy a long time ago. In a Star Wars: The Rise of Kylo Ren comic, Snoke, a clone created (and strongly implied to be puppeteered) by Palpatine, admitted to Kylo Ren not only that he respected Luke, but that he feared him. New comics recently revealed Palpatine went through even greater lengths than initially believed to combat the powerful hero.
Luke Skywalker famously avoided being tempted by the dark side and helped turn Darth Vader against his master and kill Emperor Palpatine in Return of the Jedi. However, Rise of Skywalker revealed Palpatine hadn't died and had been hiding on Exegol planning the Final Order. It was also revealed he was in control of Supreme Leader Snoke and used him as a conduit to convince Kylo Ren to join the Dark Side. Recent Star Wars comics have dug deeper into Palpatine's time on Exegol and the construction of the Sith fleet used in the final battle of RoS. The new information also provides insight into how much Palpatine feared Luke.
Star Wars: Darth Vader #11 by Greg Pak, Raffaele Ienco, Neeraj Menon, and VC's Joe Caramagna featured Vader seeking answers on Exegol, where Palpatine's "scalpel of creation" is revealed. Among his cloning experiments is Luke's hand, which Palpatine seemingly obtained after it was cut off, as well as several "Snoke" bodies. In a previous Star Wars: Rise of Kylo Ren comic by Charles Soule, and Will Sliney, and Guru-eFX, Snoke revealed he feared Skywalker - admitting that, "He has earned my respect... and my fear. He is not weak. He is misguided." Clearly, Palpatine exhausted all of his options to prepare against Luke.
In Star Wars: Rise of Skywalker, Palpatine confirmed that Snoke was an artificial being through which he had tempted Kylo Ren to the Dark Side. In the movie, Palpatine insisted, "My boy, I made Snoke. I have been every voice you have ever heard inside your head." In this context, it's hard to imagine that Snoke's words are anything but Palpatine's sincere thoughts on the Jedi who caused his downfall.
It makes sense Palpatine feared Luke after Return of the Jedi. He successfully convinced Darth Vader, his greatest warrior, to turn against him. It is somewhat surprising Palpatine - even in his guise as Snoke - would admit he feared him, however. Of course, fear is just the beginning of the path to the Dark Side, and Palpatine was also likely goading a young Ben Solo into seeing Luke as an opponent rather than a friend or mentor. Regardless, Palpatine's fear led to multiple attempts to stop Luke Skywalker, which ended with Luke dying while confronting Ben Solo. However, his reign would end when Ben and Rey - inspired by Luke Skywalker - killed him for good and stopped the Final Order - proving Emperor Palpatine feared Luke for good reason.