I think the backstory will be that Luke became the Dark Sith lord but did so unknowingly to himself.
ROTJ was always about the balance between Luke falling to the dark side, or becoming a full blown Jedi. But like the article below says even though the end of that movie made it look like Luke had redeemed himself and turned to the light, he actually may have turned to the dark side due to his actions and incomplete training.
http://www.huffingtonpost.com/rob-co...usaolp00000592
What I think happened is after ROTJ Luke went and sought out an apprentice (Kylo Ren) to continue the legacy of the Jedi. What Luke didn't realize is that with the death of Vader and the Emperor, plus with the fact that he had unknowingly turned to the dark side, he actually became the new Sith Lord and he was training Kylo Ren to be his apprentice (Follows the Rule of Two).
Additionally since Luke was going out to hide any of the sith artifacts, it actually meant he was absorbing the Sith knowledge, further cementing his place as the Sith Lord.
When Kylo Ren then fully embraces the Dark Side, Luke realizes the errors of his ways and it's when he goes into hiding.
I also think that we have a scene where Han is fighting Kylo and ask's him who he is, and Kylo pulls off his mask and says "Han, I am your Son!".