Voldemort could still be the main antagonist. He might see Harry's connection to the monsters as a threat to his own power and try to either use it for himself or destroy Harry to prevent him from uniting the monsters against him.
There could be an ancient and powerful monster that has its own agenda. It might not want Harry to become the 'King of Monsters' as it believes it should be the one in control. So it tries to manipulate events and other characters to get rid of Harry.
Another option could be a group of dark wizards who have been secretly studying and controlling monsters for their own evil purposes. They view Harry as an interference in their plans and do everything they can to stop him.
The Death Eaters could be one group of antagonists. They might see Harry's newfound power and position as the King of Valyria as a threat to their plans for wizarding world domination. So they try to stop him from fully realizing his potential in Valyria and bringing back any new magic or alliances that could defeat them.
The pure - blood supremacist wizards who don't want a half - blood like Harry as the king could be the antagonists. They might try to undermine his rule and spread false rumors about him.
One main antagonist could be the existing power - hungry rulers in Essos. They would see Harry Potter as a threat to their rule and try to eliminate him. Another could be dark magic cults in Essos that oppose any form of new leadership or change that Harry might bring.
There could be a magical creature that has been corrupted by dark magic in the North. This creature sees Harry Potter as a threat to its existence and will do everything it can to drive him out or even kill him. It might be very powerful and difficult to defeat, forcing Harry to find new ways to use his magic.
A likely antagonist could be a rogue wizard who has already made a deal with the demons and doesn't want Harry interfering. This wizard could be using the demons to carry out his revenge on the wizarding world for past wrongs. There might also be a powerful demoness who wants to use Harry as a pawn in her own power struggle within the demon realm. She could initially pretend to be an ally but then turn on him. And finally, a group of anti - Muggleborn extremists who believe that Harry's connection to the demons is a sign of impurity and try to eliminate him, thinking they are protecting the wizarding world's purity.
The pure - blood supremacist families could be the main antagonists. They might see Harry as a threat to their traditional ways of running Hogwarts. They would oppose his new rules and reforms that aim for a more inclusive and equal school. For example, they could try to undermine his authority by spreading false rumors about him among the students and their families.
There could also be a group of pure - blood wizards who look down on Harry's connection with magical creatures. They see it as beneath them and try to undermine his position as the king of magical creatures. They might spread false rumors about him and try to turn other wizards against him.
It could be other members of the royal family who see Harry as a threat to their power or position. They might try to undermine him and prevent him from claiming his rightful place as king.
The Thalmor could be great antagonists. Their strict control over magic and their anti - human beliefs could put them at odds with Harry. They might see his different form of magic as a threat to their power in Skyrim.
One possible antagonist could be a corrupt wizard in a high - ranking position in the Ministry of Magic's financial department. This wizard might be trying to cover up his misdeeds and would see Harry as a threat to his scheme.