It could be a new dark wizard or witch who has a vendetta against Harry. This person might have developed a new form of magic specifically to harm Harry, and the seizure is the first sign of this new threat.
Perhaps in this fanfiction, the bad seizure is a result of some internal struggle within Harry. He could be dealing with the immense pressure of his destiny, the weight of the prophecy, and all the losses he has endured, which manifests as a seizure.
One reason could be that Harry is under an extreme amount of stress. He has so many burdens on his shoulders in the wizarding world, like fighting Voldemort. This stress could manifest physically as a seizure, especially if he is in a tense situation in front of Snape, who is often a source of anxiety for him. For example, if Snape is about to accuse him of something he didn't do while he is already worried about other threats to the wizarding world.
In this fanfiction, the seizure could be part of a curse that was meant for Sirius but backfired on Harry. Harry starts seizing in front of Sirius, and Sirius is filled with guilt and determination to reverse the curse. He uses his knowledge of the wizarding world and his connections to search for a cure, all while taking care of Harry and keeping him safe from any potential threats.
Sirius would probably be very worried and immediately try to help Harry. He might use some basic healing spells he knows.
Snape might initially be suspicious. He has always had a distrust of Harry, so he could think it's some sort of trick. But then, as a Potions master and a teacher, he might also feel a sense of responsibility and try to help.
Among the popular bad guys in 'Harry Potter bad guy fanfiction' are the Dementors. Their terrifying nature and the mystery surrounding them make them a great subject for fanfiction. Writers can delve deeper into their origins and what makes them so evil. Another is Nagini, Voldemort's snake. Some fanfictions might give her more agency and intelligence, making her a more significant and menacing villain. And Severus Snape, while he was a complex character in the original series who turned out to be on the good side in the end, in some fanfictions he is rewritten as a true bad guy, exploring the 'what if' scenario of his character being completely evil from the start.
One characteristic of bad Harry Potter fanfiction is poor character development. Instead of staying true to the complex and well - developed characters in the original series, they might make them act out of character. For example, making Harry extremely cruel or Hermione overly ditzy. Another is a lack of consistency with the established magic system. In the original books, the magic has rules and limitations, but bad fanfiction might have magic solving every problem too easily or in ways that don't make sense. Also, bad grammar and spelling can be a big giveaway, as it makes the reading experience difficult and unprofessional.
One reason is that fans like to explore different perspectives. In the original books, Dumbledore is presented as a wise and mostly benevolent figure. But some fans want to see what would happen if he had a darker side. It adds a new layer of complexity to the story.
One factor could be a complete disregard for the established lore and characters in the Harry Potter universe. For example, if a fanfic makes Harry a completely different person with no resemblance to his original character traits, like suddenly making him extremely cruel for no good reason. It just doesn't fit with what we know and love about the series.
Well, a bad Harry Potter fanfiction could be due to bad world - building. In the Harry Potter series, the world of Hogwarts and the wizarding world is so richly detailed. If a fanfiction fails to capture that essence or creates a really inconsistent world, it's bad. For instance, if the houses at Hogwarts suddenly have completely different characteristics that don't align with what J.K. Rowling created. Additionally, if the fanfiction has a really forced or contrived plot device just to get to a certain point, like a deus ex machina that solves all the problems out of nowhere, it's a sign of a bad fanfiction.