Harry's character could be consumed by anger and a thirst for revenge in a dark fanfiction. This would make him more aggressive and less empathetic. Luna, however, might use the situation to grow in strength. She could become a sort of moral compass for Harry, but at the same time, she might have to harden herself. Maybe she starts to study dark magic not to use it but to understand it better and find ways to counter it. This would change her from a carefree observer of the magical world to a more proactive defender.
In a dark setting, their relationship might become more complex. Harry, being dark, could initially push Luna away. But Luna, being Luna, would not give up easily. She might be the only one who still believes in the good in Harry. Their relationship could turn into a battle of wills, with Luna trying to break through Harry's dark shell and Harry trying to resist her influence.
For Harry, if he goes dark, he might lose his sense of compassion. He could become more calculating and power - hungry. Tonks, on the other hand, might become more cynical. Her usually cheerful self could be replaced with a more guarded and suspicious personality as she deals with dark Harry.
She becomes more possessive. Normally, Luna is a dreamy and somewhat detached character, but in yandere fanfictions, she'll be much more clingy and protective of Harry.
One plot could be that in a dark version, Harry and Luna team up to expose a hidden, evil regime within the wizarding world that has been secretly controlling things from the shadows. They might have to use some rather unorthodox and darker magic to do so.
In these fanfictions, Harry is often made more cynical and power - hungry. His usual heroic traits are overshadowed by his darker desires. Snape, on the other hand, might be shown as more sympathetic towards Harry's new state. His strict and often cold exterior could crack as he tries to help Harry. For example, if Harry starts to dabble in dark magic, Snape may find himself torn between his duty to Hogwarts and his growing concern for Harry.
If a character has the Dark Mark, their relationships with non - Death Eaters can become extremely strained. Friends may turn against them out of fear or a sense of moral obligation. For example, if a Gryffindor was marked, their former housemates might not trust them anymore.
In 'Harry Potter Dark Luna' fanfictions, often Luna's character is developed by giving her a troubled past. For example, she might have had some experiences with dark magic early on that shaped her in a different way. This could lead to her having a more cynical view of the wizarding world compared to her original, more whimsical self.
You can start by looking on fanfiction websites like FanFiction.net or Archive of Our Own (AO3). On these platforms, you can use the search function and type in 'Harry Potter Luna' to find a wide variety of fanfictions centered around her character. Some fanfictions might explore her relationships, her adventures outside of the main story, or her unique perspective on the wizarding world.
A possible fanfiction could involve Lily Luna Potter and her time - traveling adventures. She accidentally stumbles upon a time - turning device and gets sent back to the time of the first Wizarding War. There, she has to be careful not to disrupt the timeline while also trying to learn from the events of that time to prevent a similar future catastrophe.
One great 'Harry Potter Dark Luna' fanfiction is 'The Darkened Moon of Luna'. It delves into a darker side of Luna where she has some unique powers that are a bit more on the shadowy side. Another one could be 'Luna in the Shadows', which shows her in a more mysterious and somewhat darker role in the wizarding world.