The plot might be about their discovery of a long - lost dark magic artifact. Harry and Daphne hear rumors about it and set out on a quest. Along the way, they face many challenges, like other dark wizards who also want the artifact and the Ministry of Magic's attempts to stop them. As they search, their relationship deepens. Once they find the artifact, they use it to gain more power, but also realize it has a dark side that could destroy them if they're not careful.
Well, Harry as the Dark Lord would likely have a more twisted use of his existing powers. His Patronus, for example, could be turned into a dark form, maybe a dementor - like entity that he uses to instill fear. Daphne, as the Dark Lady, might have powers that are more related to dark illusions, creating false images to deceive her enemies. Her family's pure - blood magic could be corrupted to create a new kind of dark charm that she uses to control others.
Another plot could involve them facing opposition from the traditional good side, like Dumbledore's Army remnants. They have to use their newfound dark powers and cunning to outwit their opponents. Maybe they start to realize that the line between good and evil isn't as clear as they once thought. As they fight, they also discover secrets about the wizarding world's past that make them question everything they knew.
It could be an original character created by the fanfiction writer. A witch who has been in the shadows for a long time, biding her time to rise to power.
It could be that the Dark Lord offers Harry something irresistible. Like a way to bring back his parents. Harry, being desperate to have his family back, might be tempted to join the Dark Lord in the fanfiction. After all, the love for his parents was always a strong driving force in his character, and this could be a major turning point that leads him to side with the Dark side.
The Dark Lady could be Bellatrix Lestrange. She is a powerful and fanatically loyal Death Eater. If Harry Potter were to become a fan of her in a fanfic, it might be because he is somehow tricked into seeing her as a symbol of power and freedom from the traditional wizarding rules.
Well, first, Harry would need to study the ways of the Sith. He has to understand his enemy. Then, he could combine his wizarding skills with some new knowledge. For example, he might learn how to use his magic to counter the Sith's mind - control abilities. He could also use his Patronus, which represents hope and light, to drive away the darkness of the Sith. And of course, he'd need to find the Sith's weakness, like in the way he found Voldemort's Horcruxes.
One possible plot for this fanfic is that the Dark Lady has a secret past connection with Harry Potter's family. When Harry discovers this connection, he becomes curious about her. As he delves deeper, he is exposed to her alluring but dangerous world. He might be tempted to embrace her ways as he sees some perceived 'truths' in her actions and beliefs. This would lead to a major internal conflict within Harry as he struggles between his old values and the new, darker allure.
One possibility is that Harry would be trained in dark magic from a young age. He might grow up believing in the Dark Lord's ideology, which would put him at odds with his friends from the original story. For example, he could be a formidable opponent to Hermione and Ron when they eventually cross paths.
The plot might involve the dark lord's daughter being raised in secrecy. When she discovers her true identity, she decides to use her knowledge of the dark side to actually help the wizarding world. She could start by providing inside information to Harry and his friends about hidden Death Eater lairs or secret dark spells. Along the way, she has to deal with the distrust of others in the wizarding community.
One possibility is that Harry's love for Hermione drives him to believe that the only way to protect her is by taking on the power of a Dark Lord. He might see the existing wizarding world as too corrupt or dangerous to keep her safe. So, he starts to study dark magic, gradually changing his nature. He could form new alliances with dark creatures or wizards who were previously shunned. This would put Hermione in a difficult position as she has always been on the side of good, but her love for Harry might make her question her own beliefs and try to sway him back to the light.