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, it's possible that a new group of powerful wizards or witches who oppose the idea of a Dark Lord emerging could be the main antagonists. They might not be part of the traditional good or bad sides but form a new coalition just to stop Harry. They could be from different backgrounds, some from pure - blood families who don't want the chaos a Dark Lord would bring, and some from Muggle - born communities who fear for their safety. They would use their combined magic and knowledge to try and prevent Harry from achieving his dark goals.
The Ministry of Magic could be the main antagonists. They would see Harry as a threat once he starts to show signs of becoming a Dark Lord. They would try to use all their resources to stop him, fearing that he would overthrow their existing power structure. And Harry, on the other hand, would see them as a hindrance to his new vision for the wizarding world, so there would be a great power struggle between them.
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.
His old friends, especially Hermione and Ron. They have always been on the side of good and would not stand for Harry becoming a dark lord. They would try to stop him and bring him back to the light, so they would be the main antagonists from Harry's new perspective.
The Ministry of Magic could be a main antagonist. They have a history of being discriminatory towards werewolves. They might see Harry as a threat now that he's a werewolf and try to lock him up or restrict his movements. For example, they could pass new laws specifically targeting him, like making it illegal for him to attend Hogwarts as a werewolf.
In addition to Voldemort, there might be dark wizards who are jealous of Harry's new Hanyou powers. They could team up with some of the anti - Harry Hanyous to create a powerful alliance against him. This would force Harry to make some difficult choices about who to trust and how to fight back.
The Death Eaters could still be the main antagonists. Even if Harry is Lord Peverell, they would oppose him as he is still a threat to their dark ideology.
Some dark wizards or witches who are looking for power might see Harry as a tool or a target. If they think they can harness the power of a hollow - Harry, they would try to capture him. This could lead to many battles and challenges for Harry as he grows up in this strange new form.
The Ministry of Magic could be a major antagonist. Since necromancy is considered a dark art, they would try to stop Harry at all costs. They might send Aurors after him or try to expel him from Hogwarts.
In some cases, corrupt Ministry of Magic officials can be antagonists in 'Harry Potter Dark Business Fanfiction'. They could be involved in covering up or even profiting from the dark business. For example, they might turn a blind eye to the illegal trading of dangerous magical creatures in exchange for bribes. This would make them enemies of Harry Potter and those who strive for a just and clean wizarding world.
The Hidden Leaf Village elders could be antagonists. If Naruto turns dark, they might see him as a threat to their power and try to stop him. Their actions against Naruto could further fuel his descent into darkness.
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.