Who was the Supreme Leader Snoke has long been a secret that was revealed in Star Wars: Episode 9 – The Rise of Skywalker. The accompanying book "The Illustrated Encyclopedia" for the film gives much more background information.
Supreme Leader Snoke was a mystery in Star Wars Episode 7 and Episode 8 Star Wars: Episode 9 – The rise of Skywalkers we finally learned what it was about him. But the whole story is in the accompanying book "The Illustrated Encyclopedia".
Caution! Spoilers follow here!
Kylo Ren discovers several tanks containing Snoke clones in the secret emperor's hiding place. Basically, the Supreme Leader was just a kind of puppet through which Palpatine held control over First Order without having to appear himself. In the background, he was able to build his Sith fleet and plan to recapture the galaxy. But as you can read in the accompanying book "The Illustrated Encyclopedia" for the film, there was actually more to Snoke.
It says that Snoke was an exam for Kylo Ren. Kylo should kill his master to prove that he can be considered the heir to the empire. It can also be read that Palpatine could only be revived through the power of the mysterious Sith cult, which hardly played a role in the film. The dark side of the power paired with clone technology is said to have been used.
For the film Star Wars: Episode 9 – The Rise of Skywalker, Disney's editor, Maryann Brandon, found it more interesting to be more subtle and only make hints instead of explaining everything in detail.
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