Ben Solo’s fall to the Dark Side left Luke in a state of immense grief.  Not just because Snoke (and later revealed to be Palpatine as well) had taken a hold of his nephew and he had let his sister Leia down, but because he had let the Dark Side seep into his own actions and selfishly raised his lightsaber against his own nephew, which effectively pushed Ben into the arms of the Dark Side permanently.  He couldn’t bring himself to even try to find his way to snatch Ben back.  Instead, he gave up. Why? Because he’s human. When he closed himself off from the Force, he shut the Netherworld of the Force out too and his own father couldn’t speak to him.  Luke isolated himself on Ahch-To, hoping a natural death would come soon.  But then, Rey came calling to him. 

At first, Luke was apathetic to the request that he retake the mantle given to him by taking his father’s Legacy Lightsaber. However, the persistence of young Rey made him rethink his hard stance.  When he snuck onboard the Falcon at night to mourn for the loss of his best friend, Han, it was R2-D2 that brought him to the point where he started to reconnect to the Force, and it was his father that showed up. Anakin had never given up on him and he was the first to greet him in the Force. Anakin convinced Luke to train Rey and told him not to give up on Ben like he hadn’t given up on him decades before.  Luke admitted to his father that he was not sure he could do what needed to be done.  Anakin insisted that there was something special about Rey; he could see it in the Force. He told Luke that she would have what it takes to turn the tide, but Luke was unrelenting in the fact that it would not go the way Anakin thought it would.  Luke immediately told Rey he would give her three lessons, that’s it. The next day, he woke up to start training but his hope in training Rey was that she would be able to stop Kylo Ren by killing him. He never thought that Rey would seek to save him. Luke was convinced that Ben was beyond redemption.

When he projected himself into the Battle of Crait to give the Resistance and his sister enough time to escape, he saw it himself. Ben was coming apart; the cracks ever so visible. Even when he fought his nephew, he could see that the anger he felt wasn’t what fueled the Dark Side — it wasn’t anger he felt at all, it was heartbreak.  He felt everything Ben was feeling in that moment: betrayal, abandonment, disappointment, and generally unloved. He saw the images of his mind, back to when he was a boy.  These memories were inaccurate to real facts, however.  That’s when Luke knew what had happened to his young nephew. Snoke had stripped away all the good and replaced it with conjured up falsehoods about his own family. He realized in that instant that he had more to give his nephew and that his father had been right. Like Anakin, Ben still had a spark of Light left in him.  As his life energy drained from him, he knew that as a Force Ghost he would help Ben restore these memories and that he would be a constant in his life. He told him he would see him again.  As he crossed into the Netherworld, he learned the exact nature of Ben and Rey’s connection to the Force.  He had never even heard of a Dyad, but there were others — his own master Yoda being one — that knew a great deal about it.  But will Ben forgive him?  Will he be able to bridge the mile-wide gap between his beloved sister and the father she loathes? Will Rey because the Jedi he always thought she could be?  We’ll find out in Star Wars: The Chosen One (Episode X).