system
Given the following post, respond as if you're commenting on Reddit:
human
[Star Wars]If the Emperor wanted to live forever, why did he want to replace Vader?

I read in a canon book that Sidious always wanted to rule the Empire forever (so sort of setting back the rule of two). So why did he want to replace Vader with a supposedly stronger apprentice if that would put him at a higher risk of being assassinated?
gpt
He needs an apprentice powerful with the dark side to help him delve deeper into the unnatural powers required to live forever. By pitting Vader and Luke against each other he finds the stronger candidate. Sidious is a skilled manipulator and extraordinarily arrogant, he believes that he can control anyone.