Dumbledore could be the key character. He's extremely powerful and knowledgeable. He might sense that something is wrong with Harry and use his vast magical abilities and wisdom to find a way to exorcise the demon. He could also call on the help of other powerful wizards or even use some ancient magic artifacts to save Harry.
In this fanfiction, a really engaging plot development could be the discovery of a hidden connection between the demon and Harry's past. Maybe the demon was once summoned by one of Harry's ancestors and now it's seeking revenge on the Potter family through Harry. As a result, Harry has to not only fight the possession but also uncover the secrets of his family's history to break free from the demon's hold.
A common theme is the battle for control. Harry is constantly fighting against the possessing entity for control over his body and mind. For example, in many fanfictions, he tries to resist using dark magic that the entity might force him to do.
In terms of the overall plot, it could lead to a race against time. The characters will need to find a way to exorcise Vecna from Will quickly. This might involve seeking out new knowledge, perhaps from an unexpected source. Maybe an old character from the past who has some knowledge about Vecna's possession returns to help. It also opens up the possibility for new alliances and betrayals within the group as they struggle with this new situation.
Well, one possibility is that Will starts to have visions that Vecna uses to manipulate the other characters. He might show them false images of their worst fears, leading to chaos within the group. For example, he could make Eleven see herself losing her powers and being unable to protect her friends.
Well, a frequent theme is the change in Dipper's personality. When possessed, he could become more aggressive or withdrawn, which creates a contrast to his usual curious and determined self. There may be a theme of redemption too. If Dipper does something bad while possessed, the story could be about him making amends once he's free. And sometimes, the fanfiction might use the possession as a way to introduce new powers or abilities that Dipper has to learn to control.
One of the most likely possessing entities could be Bill Cipher. He's a powerful and malevolent being in the Gravity Falls universe, and he has shown an interest in Dipper before. Another possibility is an ancient spirit or demon from the forest around Gravity Falls. The forest is full of mysteries, and it's not hard to imagine some dark entity lurking there that decides to possess Dipper.
I Possessed the Demon Empress can be read online on Qidian Chinese Network. This website provides online reading services for parts of the chapters and the entire book of I Possessed the Demon Empress. Qidian Chinese Network provided readers with a reading experience without advertisements and pop-ups.
" I Possessed the Demon Empress " was a fantasy novel written by August Eagle. The story depicted the main character, Chen Luoyang, who had transmigrated to a different world and possessed the body of the Demon Cult Master and Demon Emperor. He realized that he had become the world's top expert, but he was seriously injured and surrounded by experts. He had to pretend to be calm. The novel also mentioned that there was a limit to possession in this world. The main character could not possess the body that he had possessed again. The specific plot and development required reading the full text of the novel.
" I Possessed the Demon Empress " was a fantasy novel written by August Eagle. The story described how Chen Luoyang had transmigrated to a different world and accidentally snatched the body of the Demon Cult Master, becoming the Demon Empress. In this world full of danger and opportunities, he had to face many enemies and disciples to survive and develop. He also had a mysterious beauty to help him. The novel has been completed, with a total of 757 chapters. It can be exempted from reading on the Creation Chinese Network.