Sauron would surely be one of the main antagonists. His lust for power and domination over Middle Earth is well - known. In this fanfiction, he would see Harry Potter as a major threat to his plans.
There could also be some characters from the Harry Potter universe who turn bad in this new setting. Maybe some Death Eaters who also end up in Middle Earth and side with Sauron for power. They could use their dark magic skills to cause trouble and oppose Harry's attempts to bring peace and order to Middle Earth.
Sauron could be a major antagonist. His power and desire for domination would put him in direct conflict with Harry. Voldemort - like elements might be incorporated into Sauron's character in this fanfiction.
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.
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.
One possible plot could be that Harry Potter somehow gets transported to Middle Earth. He might use his magic skills to help in the battles against the dark forces there. Maybe he forms an alliance with the Fellowship of the Ring and uses his knowledge of facing dark wizards like Voldemort to take on Sauron.
The main antagonists could still be Voldemort and his Death Eaters. Just because Harry is being raised by Moriarty doesn't mean Voldemort's threat disappears. In fact, Moriarty might see Voldemort as either a rival or a potential ally, which would add another layer of complexity to the story.