In this fanfiction, it could start with Luke being manipulated by a powerful dark side user. This user could be a new character or an old Sith lord who has somehow survived. The dark side user could prey on Luke's emotions, like his fear of losing his loved ones or his desire to be more than just a Jedi. As he delves deeper into the dark side, he might gain new abilities but also lose parts of his former self, such as his compassion and his connection to his old friends.
Perhaps in the fanfiction, Luke discovers a dark secret about the Jedi Order that makes him lose faith in the light side. This could be something like the Jedi suppressing certain knowledge or having a hidden agenda all along. Disillusioned, he turns to the dark side, thinking it offers a more honest path. He may then start using his newfound dark powers to reshape the galaxy according to his own vision, which could be a mix of order and tyranny.
One possible plot could be that Luke is lured by the power the dark side offers. Maybe he gets frustrated with the slow progress of the light side in bringing order to the galaxy. He might see the Sith as more efficient in getting things done. For example, if the Republic is still facing numerous threats and the Jedi are tied by their code not to take extreme measures, Luke could be tempted to cross over.
In the fanfiction, Luke could be influenced by his family ties to the dark side. If he discovers some hidden connection to Vader or Palpatine that he didn't know before, it might make him question his place on the light side. He could think that joining the dark side is a way to understand his true heritage better and gain more power to protect those he loves in a different way than the Jedi would allow.
It could be the remaining Jedi. Since Luke has joined the dark side, the Jedi who still uphold the light side would be his natural antagonists. They would see him as a threat to all they stand for and would try to stop him from spreading the influence of the dark side. For example, Leia, if she still adheres to the Jedi teachings, would be in direct opposition to Luke.
Maybe in the fanfiction, Luke witnesses something so devastating that he loses faith in the Jedi way. For example, if the Jedi Order fails to prevent a great catastrophe. This could push him towards the Dark Side as he seeks a more radical solution to bring order. He might start using aggressive and unethical means, like Vader did, believing it's for the greater good.
Well, one possibility is that she gets frustrated with the slow progress of the 'good side'. Maybe she sees all the bureaucracy and inefficiency in the fight against the dark forces and decides that the dark side can offer a more direct way to achieve her goals, like a more efficient magical society. But this would be a huge shift from her original character.
In this fanfiction, it could be that Ezra is deceived by a powerful dark side figure. He might start to believe that the Jedi way has failed him and that the dark side can provide a more effective means to achieve his goals. This could lead him to make choices that gradually draw him deeper into the dark side, like using dark side powers and distancing himself from his former Jedi allies.
One possibility is that he is lured by the power promised by the dark side. Maybe he gets frustrated with the slow progress of the light side in fighting the Empire and thinks the dark side can offer a quicker solution to end the tyranny. He could then start using dark side powers like Force lightning and become a feared figure, turning against his former friends.
It could be the remaining Jedi. Since Luke is turning to the dark side, the Jedi who still uphold the light side values would be against him. They would see him as a threat to the balance they are trying to maintain and would try to stop him from fully embracing the dark side.
Sure. There's a fanfiction where Luke discovers a hidden Sith temple that has the power to reverse the light - dark side balance in a person. He enters out of curiosity but gets trapped. The temple starts to slowly turn him to the dark side. As he changes, his friends sense it and have to decide whether to try to save him or destroy him, which is a really unique take on the concept.
One possible plot could be that Luke is disillusioned with the Rebellion after some major setbacks. Maybe he discovers some secrets about the Rebellion that make him question their motives. So, he decides to join the Empire in the hope of bringing about change from within. He might start off by using his Jedi skills to gain favor among the Imperial ranks.
If Luke joins the Dark Side, it would completely change the dynamic of the Star Wars universe. His friends would be shocked and heartbroken. Leia, for instance, would probably try to bring him back. The Rebellion would lose a great hope and the Empire might gain an unexpected and powerful ally. It could also lead to a different kind of struggle within the galaxy as those who once believed in Luke now have to face him as an enemy.