The Non-Canon Expert describes what Darth Sidious saw as the aspect of Darth Vader’s training that held him back from obtaining mastery over the dark side and true power as a Sith Lord, representing a significant failure and weakness within his apprentice. This is a Legends story for Darth Vader and Darth Sidious with information found in a number of Legends sources, such as the Book of Sith, the novel «Star Wars: Death Star» by Michael Reaves and Steve Perry, and the novel «Star Wars: Rebel Force: Target» by Alex Wheeler. I hope that you enjoy it!