A plot twist could be that Rey has a dark heritage she was unaware of. When she discovers this, she feels a pull towards the Dark Side. She may struggle with this newfound knowledge at first, but eventually give in. In the story, her transition to the Dark Side could cause chaos in the Star Wars universe. The Resistance would be shocked and some might try to bring her back. However, the Dark Side could give her new powers and a different perspective on the Force. She could then start to reshape the Sith ideology, creating a new form of the Dark Side that is more in line with her own beliefs, leading to a whole new conflict in the galaxy.
She would become more ruthless. Instead of being the hopeful and somewhat naive Rey we know, she would be calculating. For example, she might not hesitate to sacrifice a few for the sake of what she believes is a greater good. Her appearance would also change. Maybe she would start to dress in darker colors, and her eyes could take on a more menacing look. Her relationships would be different too. She would probably break off ties with her old friends from the Light Side, seeing them as obstacles to her new goals.
It could be the remaining Jedi. Since Rey is turning, the Jedi who still uphold the light side would see her as a threat. They would try to stop her from causing more harm with her newfound dark side powers.
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.
In this fanfiction, it could be that Rey has some personal losses that make her turn. She might lose someone dear to her and in her grief and anger, she decides that the dark side can give her the strength to prevent such losses in the future. She then starts learning the ways of the dark side, which changes her relationships with her friends.
The consequences would be huge. The Resistance would lose one of its most powerful allies. It would also mean a new threat to the galaxy. Rey's turn to the Dark Side could inspire others to follow her, leading to a resurgence of the Sith. The balance of the Force would be severely disrupted.
In Star Wars fanfiction, Dark Rey is a fascinating concept. It allows for a different take on the character of Rey. Writers can explore her journey to the dark side, perhaps due to feelings of abandonment or a thirst for power. It gives a chance to see how her relationships, like with Kylo Ren, would change in a darker context.
If Rey turns to the Dark Side in fanfiction, one possibility is that she becomes a powerful and feared Sith. She might use her newfound dark powers to seek revenge on those who she believes have wronged her in the past. Maybe she would target the Resistance, seeing them as the ones who failed to fully understand or support her. Her connection to the Force could take a dark turn, with her using it for domination rather than protection.
Kylo Ren could still be the main antagonist even if Rey turns to the dark side. He might see her as a threat to his own power within the dark side hierarchy. Maybe he wants to keep her in check or even destroy her to maintain his dominance.
One idea could be Rey being lured to the dark side by a powerful Sith artifact. Maybe she discovers an ancient Sith holocron that shows her a version of the galaxy where the dark side rules supreme and she starts to believe that it's the only way to bring true order. Another plot could involve Rey being manipulated by a dark side user who preys on her insecurities about her past and her power.