Who could be the main antagonist in a Harry Potter summoner fanfic?

The main antagonist could be a powerful necromancer who combines the dark arts of Skyrim with some elements of Voldemort - like magic. He could be raising an army of the undead and causing chaos across Skyrim, which Harry has to stop.

It could be a new dark wizard who has a vendetta against Harry. This wizard might use a spell to stop Harry from eating, thinking it will weaken him before a final showdown. He could be someone who was once a follower of Voldemort and now wants to take his place and prove himself by getting rid of Harry.

It could be a new dark wizard who is jealous of Harry Potter's fame and success. This dark wizard targets Harry's child, thinking that by causing trouble for the child, they can somehow diminish Harry's reputation in the wizarding world. He might try to frame the child for various dark magic incidents around Hogwarts.

In an alternate dimension, the main antagonist could be a dark entity that has emerged from the very fabric of the dementions themselves. This entity could be using the Dementors as its minions, controlling them to do its bidding. It would be a powerful and mysterious foe, and Harry would have to discover its true nature and weaknesses in order to defeat it.

The main antagonist could still be Voldemort. After all, he is the main source of danger for Harry in the original series. L would then be pitted against Voldemort's dark magic and his army of Death Eaters. L would have to use his wits and any new abilities he might have in this fanfic to outmaneuver Voldemort and keep Harry safe.

It could be a new dark wizard who has emerged in Harry's new life. This wizard might be jealous of Harry's fame and want to take him down.

Another possibility is that a group of dark wizards could be the main antagonists. They see the True Longinus as a tool to bring about a new era of darkness. They band together to try and take it from Harry, and they use all kinds of dark magic and underhanded tactics. They might even try to turn some of Harry's friends against him in their quest to get the True Longinus.

The main antagonist could be an evil version of the Molecule Man. Maybe in his own dimension, he was corrupted by power and when he enters the Harry Potter universe, he wants to take over there as well. He would use his molecular manipulation powers to create all sorts of disasters, like turning the Forbidden Forest into a deadly, mutated place. Harry and his friends, along with the good - natured Molecule Man, would have to fight against this evil version.

It could be a dark wizard who wants to use the power of magical creatures for his own evil purposes. He might be trying to control them, or use their parts for dark magic spells. For example, he could be after the horns of unicorns to make a potion that gives him immortality.

It could be a new dark wizard who has somehow tapped into the power of both the wizarding world and the Narnian magic in a negative way. This wizard might want to rule both realms and is using dark magic from both to achieve his goals.