Hermione would surely be one of the main characters interacting with Harry. She might try to bring him back from the dark side, constantly arguing with him about his choices and trying to remind him of their old friendship and the good they used to fight for.
One interesting plot could be Harry's struggle with his newfound dark powers. He might initially resist the pull of the dark side but gradually find that the power it offers is alluring. For example, he could use the dark magic to protect his friends in ways he couldn't with normal magic, but at the cost of his own moral compass.
The Order of the Phoenix would be likely antagonists. Since Harry has turned to the dark side, they would see him as a threat to the peace and safety of the wizarding world. Dumbledore, if he were still alive in the fanfiction, would also oppose Harry. He would try to use his wisdom and power to bring Harry back to the light. Another antagonist could be Neville Longbottom. Neville, who has always been a symbol of resistance against the dark side, would not stand for Harry becoming a dark lord and would fight against him to protect Hogwarts and the values it stands for.
The main antagonists could be his former friends, like Hermione and Ron. Since Harry is becoming a Dark Lord, they would oppose his new, darker ways. They would try to stop him from causing too much harm to the wizarding world, which would put them in direct conflict with him.
In this fanfiction, the plot might show Harry's transformation into a Dark Lord as a result of some major betrayal in the wizarding world. For example, if the Ministry of Magic turned against him or if his friends were seriously harmed by the 'good' side. He could then start to see the power of the dark side as a means to exact revenge and reshape the wizarding world according to his own vision. He might recruit former Death Eaters or create his own dark followers.
One interesting plotline could be how Harry's relationships change. His friends might initially try to save him from the dark path, leading to intense emotional confrontations. For example, Hermione could use her vast knowledge to find a way to reverse whatever made Harry turn dark, while Ron might be more impulsive in his attempts to bring Harry back. Another plotline could involve the wizarding world's reaction. The Ministry of Magic would likely see Harry as a huge threat and try to stop him, which could lead to all - out war. And Harry himself might have new powers or a different understanding of magic as a dark lord, which could introduce unique magical elements into the story.
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.
Well, when this happens, there could be a major upheaval in the magical world. Harry's powers might develop in unexpected ways, and his role in the fight against evil could take a wild turn. Also, his friendships and alliances could be redefined.
His friends like Ron would be extremely shocked and confused. Ron, who has always been by Harry's side, would find it hard to accept this new Harry. He might initially try to fight Harry, thinking he has been corrupted. Other characters in Hogwarts, such as the teachers, would be divided. Some might see it as a way for Harry to protect Hermione and understand his motives, while others would be strictly against his new path. The Order of the Phoenix members would also be split, as they have put so much faith in Harry as the 'Chosen One' to fight against dark forces, not become one.
The story usually takes a wild turn with lots of unexpected plot twists. Harry's relationship with the Dark Lord leads to power struggles and internal conflicts.