In a 'Harry Potter Heir of Sparda' fanfiction, the main antagonist could be a combination of Voldemort and a corrupted figure from the Sparda lineage. Voldemort is still hell - bent on ruling the wizarding world. The corrupted Sparda figure might be jealous of Harry being the heir and wants to take the powers for themselves. They could form an alliance, with Voldemort providing the wizarding world knowledge and forces, and the corrupted Sparda figure bringing in their own dark powers from their world. This combined force would be a huge obstacle for Harry to overcome.
In a 'Harry Potter Heir of Sparda' fanfiction, the plot might involve a mysterious connection between the wizarding world and the world of Sparda. Harry could be led to an ancient prophecy that links him to the Heir of Sparda. As he delves deeper, he meets characters from the Sparda universe who teach him how to harness his new powers. He then uses these powers not only to defeat Voldemort but also to protect the entire magical world from threats that were previously unknown.
Maybe a jealous student or a group of students who feel threatened by Harry's new - found status. They could be from pure - blood families who think Harry doesn't deserve to be the heir of so many houses. They might start spreading rumors and causing trouble for Harry at Hogwarts, and eventually, their actions could lead to more dangerous situations as they side with darker forces out of envy.
Voldemort would most likely still be the main antagonist. Even with Harry being the heir of Darth Revan, Voldemort's goal of ruling the wizarding world remains the same. His dark magic and army of Death Eaters would pose a significant threat to Harry and his friends.
It could be Voldemort. Even though the Darth Nox element is introduced, Voldemort might see Harry's new connection as a threat to his own power. He could try to either use Harry's new powers for his own gain or destroy Harry to prevent him from becoming too powerful with this strange new heritage.
One possible main antagonist could be Voldemort still. Even though Harry is now under Sparda's protection, Voldemort would see Harry as a threat and would try to get rid of him. Another could be some of the demons from Sparda's world who see Harry as an intruder or a threat to their plans in the new world order that Sparda is trying to create with Harry by his side.
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.