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 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 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.
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 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.
The Ministry of Magic could turn into an antagonist. If Harry's new cold and strong ways are seen as a threat or too radical, they might try to stop him. This could lead to a conflict between Harry and the wizarding government.
Perhaps the very magic that made Harry a god has a sort of negative side - effect. This could manifest as a shadowy entity within Harry that tries to take over his mind and use his powers for evil. So, in a way, this internal struggle could be the main antagonist in the story.
One potential main antagonist could be a powerful pirate captain who sees Harry as a threat to his control over the seas. Maybe this captain has some dark magic powers as well.
Some jealous students at Hogwarts could also act as antagonists. They might be envious of Harry's newfound allure and use underhanded tactics to try and make his life difficult.
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.
In this fanfiction, it could be that some of the Slytherin students who side with Voldemort become the main antagonists. They might be jealous of Harry's new power and try to expose him or cause trouble for him. For example, they could set traps in the forest hoping to catch Harry when he's in his deer form.