here is the extended explenation (and the reason for why the scene was scrapped) "all of it originating from Wookieepedia":
In 2011, Revan was slated to appear in Ghosts of Mortis, an episode of the third season of the Star Wars: The Clone Wars television series. Revan would have appeared as a Sith Lord alongside Darth Bane as advisors to the Son, but they were cut from the episode in late production. The show's director, Dave Filoni, was apprehensive of using the scene, and the scene was removed at George Lucas's request because it conflicted with Lucas's view on the Force. The scene made it to the animatics stage and could be viewed in the bonus content on the Blu-ray edition of Star Wars: The Clone Wars The Complete Season Three. The scene is set in the Well of the Dark Side on the planet Mortis, where the powerful Force wielder known as the Son confronts dark side spirits about the recent death of his sister, the Daughter, at his hands. The spirit voices tell the Son to embrace his pain and resentment, and when he questions their identities, giant apparitions of Darth Bane and Darth Revan appear to the Son, proclaiming their intent to ensure the dark side's supremacy. Speaking of another's existence who will control the galaxy, Revan and Bane warn the Son that control over Anakin Skywalker, the Chosen One, is the key to the Son's success.