Voldemort or his Death Eaters are obvious possible culprits. They've always wanted Harry dead, and poisoning could be a more insidious way to get rid of him compared to a direct attack. Another possibility could be a jealous student at Hogwarts. Maybe someone is envious of Harry's fame and power, so they poison him out of spite. It could also be a wizard from a rival wizarding school who sees Harry as a threat to their own school's reputation.
It could be a disgruntled former Death Eater who wants to prove himself to Voldemort by taking out Harry. Or perhaps a pure - blood supremacist who thinks Harry, as a half - blood, is a stain on the wizarding world. There's also a chance it could be a magical creature that has been corrupted or controlled by dark magic to poison Harry. For example, a poisoned Niffler might be used to pass the poison to Harry without raising suspicion.
A possible culprit could be a secret agent of the Ministry of Magic who has been bribed or blackmailed into poisoning Harry. This person might be working for someone who wants to create chaos in the wizarding world. Another option is a new dark wizard or witch who has emerged from the shadows and sees Harry as the first target in their quest for power. Or it could be an old enemy of Harry's parents who has finally found an opportunity to take revenge on their son.
It could be the Death Eaters. They are always looking for ways to cause chaos and undermine the wizarding world. Hogsmeade, being a place full of both students and ordinary wizards, would be a prime target for them to strike fear into the hearts of everyone. Another possibility could be some rogue dark wizards who are not directly affiliated with Voldemort but have their own evil agendas.
One possible plot could be that Harry has been influenced by a powerful dark object. This object slowly corrupts his mind, making him believe that Hagrid is a threat to his new - found dark power. So, he decides to poison Hagrid. Another plot could be that Harry has been misled by a dark wizard who convinces him that Hagrid has betrayed him in some way, and in his anger, he resorts to poisoning.
Well, one possible plot could be that Harry's friends race against time to find an antidote. Maybe Hermione uses her vast knowledge of magic to research ancient spells and potions that could counteract the poison. Another could be that the poison has some strange side - effects on Harry, like giving him temporary new magical abilities while also slowly killing him, and he has to figure out how to use these new powers to save himself. Or perhaps the poisoning is part of a bigger conspiracy within the wizarding world, and Harry's poisoning is just the first step in a plot to take over Hogwarts or the Ministry of Magic.
It could be Draco Malfoy. He has had a long - standing rivalry with Harry. Maybe in this fanfic, he finally decides to take extreme measures out of envy or pressure from his family.
One possible plotline could be that Peter is poisoned by one of his enemies, like the Green Goblin. He then has to struggle to find an antidote while still trying to keep his identity a secret. Maybe he uses his scientific knowledge to analyze the poison and find a cure, all while fighting off the effects of the poison.
It could be a jealous student. Maybe someone who has a crush on the same person Harry is attracted to in the story. They see Harry as a rival and figure that by poisoning him with a love potion, they can make him look foolish or get him into trouble.
It could be a rival guild. They might see Lucy as a threat because of her powerful celestial spirit magic. By poisoning her, they hope to weaken Fairy Tail. For example, a guild that has always been envious of Fairy Tail's reputation and power.
Perhaps an enemy from a rival group. They might see Sakura as a threat to their plans, so they poison her to get her out of the way. Another possibility is a character who has been secretly harboring a grudge against her for a long time, and finally decides to take action by poisoning her.
The poisoned apple could be a gift sent by a mysterious figure to one of the characters, like Iron Man. When he touches it, it starts to release a dark magic that affects his suit's technology. Harry Potter then detects the magic and tries to find a way to reverse the poison's effects.
There's a possibility that it could be a new character introduced in the fanfiction. This character might have been orphaned and raised in a muggle family, not knowing their true heritage. But as they start at Hogwarts, strange things start to happen and it's revealed that they are the heir to one of the founders. It could also be an existing character like Neville Longbottom. He has shown great courage and loyalty, and there could be a backstory written where he is actually related to one of the founders through a long - forgotten branch of the family.