Star Wars: The Rise of Skywalker - essentially sums up the climax of the sequel trilogy. The resurrected Emperor was responsible for the rise of the First Order, and he was making his final attempt to take over the galaxy.
The sequels left a lot of unanswered questions. It's been left to tie-ins to explain exactly how Palpatine returned, and they've done a creditable job. More curious, however, is the fact Luke Skywalker - generally seen as the greatest Jedi Master of all - was clearly unaware of Palpatine's resurrection. That's particularly disturbing given the Emperor seduced Ben Solo to the dark side under Luke's watch. There is, of course, a parallel between Luke's blindness to the Sith and that of the prequel-era Jedi, who had no idea a Sith Lord had been Chancellor of the Republic until it was too late. But it's rather surprising to see Luke make the same mistakes as the prequel Jedi, especially given he'd already lived in a time when the dark side was dominant, so he should really have recognized the signs.
Adam Christopher's novel Shadow of the Sith finally explains how Palpatine hid from Luke Skywalker. The Jedi Master did indeed sense the rise of the dark side, and indeed he was even drawn to the ancient Sith redoubt of Exegol. But he tricked Luke into believing this was connected to the ancient Sith spirit of Viceroy Exim Panshard, whose spirit had been contained within his mask. Then, after Exim Panshard was defeated, the Emperor used the Force to conceal his presence in the Force. Luke perceived his connection with the Force as somehow empty; "like the disturbance had been the tolling of a great bell, and now that it was gone, the silence it left behind was almost more deafening." Luke assumed this would with time - but, of course, it would not.
Palpatine probably used the same technique on the Jedi in the prequel era - allowing him to act with impunity, even using the dark side in the Jedi Temple itself. In Luke's case, the Emperor remained physically distant, but he monitored the Jedi carefully, and worked hard to seduce Ben Solo over the next few years. Tie-ins have hinted Palpatine only lifted the veil on that fateful night when Luke entered his nephew's room and probed him with the Force; the darkness Luke sensed was too vast to be contained within Ben alone, but rather was a sudden awareness of the Emperor's spirit in his presence. Little wonder Luke reacted without thought, triggering his lightsaber - just as Palpatine had hoped he would do.
Luke Skywalker's quest for Exegol had the potential to ruin all the Emperor's plans. Had Luke found the Sith Wayfinder and discovered Exegol, he would have discovered the Sith cultists there - and, with the power he inherited from his father flowing through his veins, Luke would have defeated the Sith Eternal. Unfortunately for the galaxy, yet again Palpatine's genius allowed him to outmaneuver the Jedi, and the greatest villain of Star Wars did indeed return.
