Star Wars has explained why Luke Skywalker didn't realize Palpatine was alive in the sequel trilogy era. "Somehow, Palpatine returned." That sentence — said by Poe Dameron in 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.
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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
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