Who might be the main antagonist in a 'Harry Potter lifts Excalibur' fanfiction?

Well, perhaps in the fanfiction, Harry Potter is on a quest outside of the wizarding world and stumbles upon Excalibur. The sword recognizes his pure heart and heroic nature, allowing him to lift it. This event could then draw the attention of both the wizarding and Muggle worlds. Harry would then have to deal with the new responsibilities that come with wielding such a powerful and legendary sword, all while still having to deal with the threat of Voldemort.

Perhaps there's a member of the royal court who is secretly envious of Harry's power and tries to undermine him. This person could be plotting against Harry and trying to make the queen believe that he is a danger rather than an asset. They might spread false rumors or create false evidence to get rid of Harry.

Maybe the main antagonist is a dark force that has been slowly taking over the alliances from within. Harry notices this and decides to cancel the alliances to prevent the dark force from spreading further. This dark force could be something like an ancient evil that has awakened and is using the alliances as a way to gain a foothold in the wizarding world.

It could be a new player on the team who is jealous of Harry's skills and popularity. This new player might spread lies about Harry, turning the other team members against him. So the new player would be the main antagonist in this scenario.

It could be a new dark wizard who has been secretly searching for the Death Stone. Maybe someone who was in the shadows during Voldemort's time and now sees an opportunity to rise with the power of this stone.

It could be a new dark wizard we haven't seen in the original series. Maybe someone who has a vendetta against Hogwarts and manipulates Harry into doing the dirty work of destruction.

Dumbledore could be seen as the main antagonist from Harry's new perspective on the dark side. Dumbledore represents all that is good in the wizarding world, and if Harry has turned to the dark, Dumbledore would be the one trying to stop him, to bring him back to the light. So in this context, Dumbledore becomes the opposition that Harry has to face.

It could be Voldemort still. Since Harry is dabbling in dark magic by making a Horcrux, Voldemort might sense this and try to use it to his advantage. He could either try to steal Harry's Horcrux or manipulate Harry further into the dark side.

It could be the wizarding society itself. If Harry gives up his humanity, the wizarding society might turn against him. They could become the antagonists as they try to stop Harry from causing more harm. The Ministry of Magic could lead the charge, fearing that Harry's new state will disrupt the peace and order they have been trying to maintain. They might use all means necessary, including dark magic if they have to, to subdue Harry.

Perhaps it's an old enemy from Harry's past, like Draco Malfoy who has turned to the dark side completely. He blames Harry for all his misfortunes and decides to cage him without his glasses as a form of cruel punishment.