Star Wars: The Fallen Brother (2016) online

Taking place during and after the events of both Empire Strikes Back and Return Of The Jedi, Yoda trains two brothers the ways of the Jedi in case Luke Skywalker fails to defeat Vader. When something goes wrong, one of the brothers turns to the dark side and becomes the new Sith after Vader's death. It is up to the Sith's brother to stop him from wreaking havoc upon the galaxy