Regulus Black Gryffindor Fanfiction: Who could be the main antagonist?

Perhaps an old enemy who has infiltrated Gryffindor in disguise. They use their influence to make the Gryffindors believe that Harry is a danger. They could be a Death Eater or a follower of Voldemort who has found a way to get into Gryffindor and manipulate the students' minds, causing them to betray Harry.

A dark force that has the ability to influence the minds of Gryffindor members. This could be a powerful and ancient magic or a new form of dark magic. It turns the house against Harry without them fully realizing it, making the collective Gryffindor house, under this influence, seem like the antagonist.

One idea could be that Regulus Black, despite being a Slytherin in the original story, has a secret admiration for Gryffindor values. So, in the fanfiction, he could start secretly taking part in Gryffindor - like adventures. For example, he might help Gryffindor students in a fight against some dark forces in the Forbidden Forest.

Maybe it's a new Gryffindor student who has been influenced by the wrong people. This new student doesn't really know Harry well and is easily convinced by some anti - Harry propaganda within the school. So they become the main antagonist in the story and lead the attack on Harry.

Perhaps an ancient evil that Gryffindor once sealed away is starting to break free. This entity sees the descendant as a threat because they have the potential to reseal it. So, it tries to corrupt the descendant or stop them in any way possible.

Maybe the Akatsuki as a whole. They are a group of powerful and dangerous ninjas. The Black Knight's presence might disrupt their plans to collect all the tailed beasts. They would see him as an obstacle and try to eliminate him. For example, Pain might think the Black Knight is a threat to his vision of peace through pain and send his subordinates to fight him.

An old rival of T'Challa's from his training days could be the main antagonist. This person was always jealous of T'Challa's status as heir to the throne and the future Black Panther. Now, he has found a way to enhance his own abilities and wants to prove that he is the better warrior by taking down T'Challa and claiming the Black Panther mantle for himself. He would know T'Challa's fighting style well, making him a very challenging opponent.

A new character could be the main antagonist. For example, a powerful alien who has heard of Goku Black's reputation and wants to prove himself by defeating the reincarnated warrior. This alien could have unique powers that are a challenge for Goku Black to overcome, like the ability to nullify ki attacks.

Perhaps a powerful sorcerer who wants to capture the black horse for his own dark purposes.

The Death Eaters could be antagonists. Even though Regulus turned against them, they might not forgive his betrayal and try to harm him or the school where he teaches. They could send spies or launch attacks to get revenge.