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.
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.
There could be new characters introduced, like old family retainers of the noble family Harry is now part of. Maybe an old wizard who serves as an advisor to Harry on matters of the lordship. And of course, some antagonists such as rival lords or wizards who don't want Harry to gain too much power.
Obviously, the son of the Dark Lord would be a main character. He would be central to the story, with his actions and decisions driving the plot. Harry Potter himself might also be a key character. He could be a mentor figure to the son, guiding him away from the dark path. Hermione and Ron could also be involved, perhaps initially being suspicious of the son but later coming to understand him.
The obvious antagonists would be the Death Eaters. They are Voldemort's followers and are often causing trouble in the wizarding world. In this fanfiction, they might be the ones who have put Cedric in danger, either to lure Harry or for some other dark purpose.
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.
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.
Dark wizards or witches who are on a power - hungry spree could be the antagonists. They might see Daphne as an easy target or a way to get to Harry Potter himself. So they kidnap her to lure Harry into a trap.
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.
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.