Darth Sidious, if not the most powerful, was one of the most powerful Sith Lords of all time. He was also known as Sheev Palpatine, Chancellor of the Republic during the Clone Wars. While revealing himself to the public as the Dark Lord of the Sith, he was the Emperor of the Galactic Empire. I have seen in Youtube comments and on Reddit that the Emperor feared Luke Skywalker for reasons I do not know. So why does Darth Sidious fear Luke Skywalker?
Do Legends, such as the Return of the Jedi novelization, mention this?