The plot could be that Harry, raised by demons, has unique powers that other wizards don't. For example, he might be able to communicate with dark creatures more easily. The demons could be using Harry as a tool to infiltrate the wizarding world. When he gets to Hogwarts, he starts to question his upbringing as he sees the kindness and different ways of magic there. He has to decide whether to stay true to his demon - raised self or embrace a new way of life.
Perhaps the story could show Harry being isolated from the wizarding world he was meant to be a part of. Since demons are raising him, they might keep him away from Hogwarts and the positive influences there. He could develop powers that are a mix of wizardry and demonic abilities, which would make him a very different kind of character in the fanfiction universe. He might also have a different view on good and evil, as demons' moral compass is not the same as that of humans or wizards.
One possibility is that Harry Potter, in this fanfiction, might be exposed to a whole new set of dark magic and powers that demons possess. The demons could see his potential and decide to raise him in a way that is completely different from the wizarding world he knew. Maybe they would teach him to use his magic more aggressively or in ways that are considered unethical in the normal wizarding community.
Well, it could be a really wild plot. Maybe Harry discovers he has the ability to use his magic to make demons sing, and he uses this as a way to defeat Voldemort. For example, the demons' singing could have some kind of magical effect that paralyzes the dark forces.
In the 'Harry Potter Saved by Demons' fanfiction, perhaps the demons are not as evil as they seem in the traditional sense. They might be outcasts from their own demon realm. When they come across Harry, they recognize his power and the danger he's in. They decide to save him because they believe that by doing so, they can change the fate of both the wizarding world and their own world. So they use their unique demon abilities to keep Harry safe from harm.
One possible plot could be that Calypso, with her magical abilities, imparts a different kind of magic knowledge to Harry. Maybe she teaches him more nature - based magic, which is different from what he would learn at Hogwarts. Harry might grow up in a more isolated and peaceful environment with Calypso, away from the prying eyes of the wizarding world in Britain. This could lead to him having a different perspective on magic and his role in the fight against Voldemort.
The fanfiction might explore how Harry's relationship with the wizarding world is different. Growing up in Potter Manor, he could be seen as a more prominent figure from a young age. The manor itself could hold hidden chambers filled with challenges and rewards. Harry could stumble upon a chamber that tests his Potter lineage, like a series of magical trials. This would not only strengthen his magical abilities but also deepen his connection to his family name and the responsibilities that come with it.
One possibility could be that Harry Potter grows up with a strict military - like discipline on Kamino. He might be trained in unique combat skills that are different from what he learned at Hogwarts. He could also form unexpected alliances with the Kaminoans, and use their advanced cloning technology to his advantage in the wizarding world battles.
One possibility is that Harry would grow up to be a very different person. Dementors are creatures of despair, so he might be more withdrawn and less trusting. He could also have some very dark powers, having been exposed to the dementors' influence for so long.
Perhaps in this fanfiction, the goddess protects Harry from Voldemort right from the start. She could raise him in a hidden, magical realm where Voldemort can't reach. Harry would then grow up with a sense of security and a different kind of magical upbringing. He might learn to control his magic in ways that are more instinctual and natural, as influenced by the goddess.
The plot could involve Harry being trained in unique ninja skills by the Akatsuki members. He might learn their combat techniques and use them against the dark forces in the wizarding world. For example, he could use Akatsuki - style stealth to infiltrate Voldemort's lairs.