Star Wars: The Rise of Skywalker remains a divisive movie, with Emperor Palpatine's shocking resurrection not quite the epic moment that Lucasfilm and filmmaker J.J. Abrams likely hoped it would be.
Episode IX did a poor job of explaining how Palpatine had somehow returned. Ultimately, we were able to piece together a vague series of events that included him transferring his essence into a waiting clone body, unable to contain his powerful Sith form. The villain spent decades trying and failing to create a suitable form and managed to manipulate Kylo Ren by possessing Supreme Leader Snoke.
We're still not sure whether Snoke was another failed clone or a being from elsewhere in the Galaxy; the former seems more likely, but with fan service such a priority in The Rise of Skywalker, explanations weren't much of a priority for Abrams, it seems.
A new canon novel, Star Wars: Master of Evil by Adam Christopher, has just been released, and it sheds new light on why Palpatine was in such a weakened state when we found him in The Rise of Skywalker.
While inhabiting a failed clone certainly didn't help, it seems the loss of Darth Vader hit the Emperor harder than expected, as this excerpt from Master of Evil explains (via SFFGazette.com):
"The power of the dark side is the truth, but it is not a truth to be shared. Its secrets are to be gathered, hoarded, kept for the self. That is how it is meant to be. The master’s strength and power come from the anger and fear of his apprentice. The master uses that power, absorbing the resentment, fueling the fire, focusing his power and his grip on the dark side. It is the apprentice that makes the master, not the master the apprentice."
This explains why the immensely powerful Anakin Skywalker was so appealing to Palpatine as an apprentice, and why he kept him by his side even after the former Jedi became more machine than man as Darth Vader.
When Vader turned on his Master, the "Rule of Two" was broken, and without his apprentice's power to leech from, the villain was weakened. As for why he used Snoke as an intermediary rather than taking Ben Solo as his apprentice, Palpatine likely feared another betrayal in his frail form.
That left Palpatine forced to rely on getting Rey to kill him in anger so he could take over his "granddaughter's" body. Ultimately, it was her and Kylo Ren's Force Dyad that allowed the Emperor to regain some of his lost strength, but it appears the Rule of Two is no more in the Galaxy now that the villain has fallen.
Where that leaves the Sith as we move into an era of storytelling beyond Star Wars: The Rise of Skywalker remains to be seen.