Who could be the main antagonist in a 'Harry Potter and Merlin's Castle' fanfiction?

One possible plot could be that Harry discovers a hidden passage in Hogwarts that leads to Merlin's Castle. There, he finds ancient spells and artifacts that help him in his fight against Voldemort. Maybe he even meets a ghostly form of Merlin who gives him advice.

Morgana could be a great antagonist. She might see Merlin's sister as a threat to her own plans for Camelot. Maybe she discovers that Merlin's sister has a power that could undermine her magic or her influence over the court.

It could be a rogue Death Eater who survived Voldemort's fall and blames Harry and his sister for the downfall of their cause. This Death Eater might be more cunning and powerful than the others, and he targets Harry's sister specifically because she represents a new threat to their dark plans. He might try to use her as a pawn to draw Harry out and then destroy them both.

Well, it could be Agravaine. He's always looking for ways to gain power and influence in Camelot. If he finds out about Merlin as a borrower and the unique situation, he might try to use it to his advantage. He could either try to expose Merlin to gain favor with Uther or try to control Merlin and use his magic for his own dark purposes. Also, there could be a group of anti - borrower humans in the castle. They might be superstitious or just plain cruel and they could be the main antagonists as they try to hunt down Merlin and the other borrowers.

Perhaps a long - lost relative of the Potter family who feels they were unjustly excluded from inheriting the manor. This relative could be jealous of Harry's status and the wealth associated with the manor. They might use dark magic to try and claim what they think is rightfully theirs, leading to a battle of magic and loyalty within the Potter family and among Harry's friends.

A new dark wizard or witch could be created specifically for this fanfiction. This character could be interested in using the Huntsman's skills for their own evil purposes. If the Huntsman refuses, they become enemies. This antagonist could have powers related to dark forest magic, something that the Huntsman is familiar with but in a more malevolent form.

Voldemort could still be the main antagonist. Even if Demetri has some sort of connection or change in the fanfiction, Voldemort's overarching evil plans would still be a major threat to both Harry and Demetri, especially if Demetri tries to deviate from the Death Eaters' path.

The minotaur itself could be the main antagonist. It's a powerful and dangerous creature, and if it's been set loose in Hogwarts or is guarding something that Harry needs, it would be a major obstacle for him to overcome.

The main antagonist could be a dark wizard who has made the graveyard his lair. He uses the dark energy from the tombstones and the restless spirits there to power his dark magic. Harry stumbles upon this wizard's activities and has to stop him.

A new dark wizard or witch could be the main antagonist. This character, jealous of Harry's fame and power, creates a situation where Harry is framed for a heinous crime. They then ensure his arrest so that they can take over the role of the 'chosen one' in the eyes of the wizarding public and start spreading their own brand of dark magic.