Who could be the main antagonist in 'Harry Potter King of the Spiders Fanfiction'?

Maybe it's a rogue group of wizards who fear Harry's power over Dementors. They might think that Harry is going to use the Dementors to take over the wizarding world in an unjust way. So they band together to try and stop Harry before he can fully embrace his role as the King of Dementors.

Another option could be a corrupted version of Merlin. What if he was turned evil by some dark magic? He would know all the secrets of both the Arthurian and wizarding worlds. He could use this knowledge to create chaos and try to rule over both. Harry and the good knights of Arthur would have to find a way to reverse his corruption.

It could be a dark fairy who is jealous of the Fairy King's alliance with the wizarding world. This dark fairy wants to destroy the connection and take over both the fairy realm and the wizarding world.

In this fanfiction, it could be a group of corrupted magical nobles who oppose Harry's rule. They are used to having all the power in the alternate universe and don't want to give it up to an outsider like Harry. They could be plotting all sorts of schemes to undermine Harry, from spreading false rumors to outright attacking him and his supporters.

One possible interesting plot could be Harry forming an unexpected alliance with the spiders. Maybe he discovers a way to communicate with them on a deeper level and uses their knowledge of the Forbidden Forest to his advantage in a battle against the dark forces. For example, the spiders could lead him to a hidden magical artifact that has the power to defeat Voldemort.

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.