To expand on this: Kylo Ren could be Luke's apprentice. The two go to Dagobah to train without interference. Kylo goes into the cave and turns to the Darkside. A battle ensues which Kylo loses. He then takes the ship they both arrived on and leaves Luke stranded with only R2 for company. Then years later, Luke feels a disturbance through the force (his old lightsaber being found, Kylo being a Dark Jedi/Sith & leading the first order) and contacts the person(s) who have his old lightsaber for help with force telepathy so they he may confront his former apprentice in an attempt to sway him from the Darkside like he did with Vader.