Who could be the main antagonist in a 'Harry Potter turned into a merman' fanfiction?

A power - hungry merman who wants to rule over all the mer - people. He sees Harry as a threat because Harry has some special powers or a claim to the throne that he doesn't want to share.

It could be Voldemort. He might see turning Harry into an elf as a way to weaken him. Since elves are generally seen as lower in the wizarding hierarchy, Harry in this form would be less of a threat to Voldemort in the eyes of many wizards.

It could be Voldemort. He might see Harry's new snake form as a way to either use him for his own evil purposes or as a threat to his power. Since snakes are often associated with Slytherin and dark magic, Voldemort might try to control Harry.

Well, it could start with Harry being cursed by a dark spell during one of his adventures near the Black Lake. Suddenly, he starts to feel strange sensations and discovers his legs are fusing together and turning into a merman's tail. He has to adapt to this new form. Maybe he discovers he can communicate with the merfolk in a different way now and they have some secrets about the wizarding world that only he can learn in this form.

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.

Maybe there could be a new antagonist, a dark shapeshifter who is jealous of Harry's power. This antagonist could be an old wizard who has been in hiding for centuries, waiting for the right time to emerge and take over the wizarding world. He sees Harry as a threat to his plans and tries to eliminate him.

Another possibility is a corrupted member of the Ministry of Magic. They might be trying to use Harry as a scapegoat to gain more power within the Ministry. They could be working with the Death Eaters or acting on their own, but they would be a significant obstacle for Harry as he tries to stay alive and free while on the run.

Perhaps a new dark wizard or witch who has infiltrated the Ministry of Magic and made sure Harry was sent to Azkaban. This antagonist could be working towards a bigger plan to take over the wizarding world and sees Harry as a major threat even when he's locked up.

It could be Voldemort. He might see Harry's new waterbending ability as a threat and try to find a way to counter it. Maybe he would seek out dark magic related to fire to oppose Harry's water powers.

It could be a new dark wizard who has risen to power. This wizard might see Harry as a threat to his plans, so he manipulates the situation to get Harry exiled.