One possible motive could be revenge. If someone he loves, like Ginny or Hermione, is seriously hurt or killed by the wizarding community's negligence or by someone who is supposed to be on the good side. Harry's love for them could turn into a burning desire for vengeance, and he might see the dark side as a way to get that revenge, as they are not bound by the same moral constraints as the good wizards.
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.
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.
One plotline could be that Harry is influenced by the Dark Lord's ideology from an early age. Maybe he discovers some 'truths' as he sees them that make him believe the Dark Lord's way is the right one. For example, he might think that the wizarding world's current order is corrupt and only the Dark Lord can fix it.
The new Dark Lord in this fanfiction could be a former Hogwarts student who was bullied and turned to the dark arts for revenge. They could start gathering followers among those who feel disenfranchised in the wizarding world. Their goal might be to rewrite the laws of magic to suit their own dark desires. They would likely oppose Harry Potter and his friends, leading to epic battles and tense stand - offs.
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.
Maybe she could be motivated by power. The Dark Lord might have promised her a high position in his new world order. She might also be disillusioned with the wizarding world's current state and think that the Dark Lord could bring about a better change, even if it means using dark magic.
It could be a powerful, ancient Titan who was banished long ago and has now returned for revenge. Maybe Kronos has an even more evil and powerful form that he takes on, becoming the Dark Lord. He wants to destroy the new order of the gods and demigods that has been established since his last defeat.
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.