It would be very different from the normal Harry Potter stories. It might involve dark magic and a deviation from the moral code of the wizarding world. Maybe Harry would be in a desperate situation and make a really bad choice to summon the devil.
A 'Harry Potter summons the devil' fanfic would likely be a very dark and controversial piece. It could involve Harry delving into the darkest corners of magic that are never explored in the original series. The devil in this context might be some sort of extremely powerful and malevolent entity. The story might show how Harry's life spirals out of control as he tries to deal with the consequences of his actions. It could also explore how the wizarding community reacts to such a heinous act, with some perhaps being tempted by the power the devil might offer and others firmly against it. This type of fanfic would be a far cry from the more light - hearted and morally - centered original Harry Potter stories.
I'm not entirely sure if there is a specific 'Harry Potter summons the devil' fanfic. Fanfiction is a huge realm with all kinds of ideas. But in the official Harry Potter universe, magic is more about good - evil magic like summoning the devil is not in line with the main story. It could be a very niche and probably not very popular idea among the more traditional Harry Potter fans.
It could be a very dark and twisted take. Harry might grow up with a completely different set of values, perhaps being more ruthless and less empathetic. His magic could be used in more malevolent ways, and his relationships with the wizarding world would be severely strained as he's been influenced by the devil.
One possible plot could be a wizard accidentally stumbles upon a Devil Fruit while on an adventure in a far - off land. He eats it and gains the power, say, of the Gomu Gomu no Mi (Rubber - Rubber Fruit). Then he has to hide this new power from his Hogwarts friends and teachers at first because he's not sure how they'll react. As he gets used to the power, he starts using it to his advantage in Quidditch, like stretching his arms to catch the Snitch from a great distance. But then dark forces in the wizarding world become interested in his unique power and he has to fight them off with a combination of his wizardry and Devil Fruit ability.
Well, in such a fanfic, Harry might summon a servant with unique powers. Maybe the servant could help him in his battles against Voldemort. For example, the servant could have the ability to detect dark magic from afar, allowing Harry to be more prepared for attacks.
It could be a house - elf. House - elves are known for their loyalty and their ability to do various tasks. They could be very useful to Harry, like helping him keep his dormitory clean or even spying on the Death Eaters.
In some fanfics, an order summons might be a magical charm that appears as a constellation in the sky, visible only to those who are part of the 'Order'. For instance, if the Order is reforming to fight a resurrected Voldemort - related threat, the summons could call wizards from all over the wizarding world to a hidden location like the Room of Requirement.
One idea could be that Harry discovers an ancient spell in the restricted section of the library that has the power to summon Echidna. He summons her in a time of great need, like when Hogwarts is under a new dark threat. Echidna, being a mythical and powerful creature, could offer her unique knowledge or abilities to help Harry and his friends fight off the danger.
The plot might involve Harry using his magic in an unorthodox way to summon Cthulhu. Maybe he stumbles upon an ancient spell in the restricted section of the library. Once summoned, Cthulhu could bring chaos to the wizarding world. The dark and eldritch powers of Cthulhu would be in stark contrast to the more traditional magic in Harry Potter. This could lead to a struggle between the forces of Hogwarts and the indescribable horror of Cthulhu.
Many fanfics depict the order summons as a form of enchanted parchment. The sender writes the details of the summons on the parchment. Once it's complete, the parchment magically replicates and finds its way to each member of the order. The parchment might have a special seal or symbol that only the members recognize, ensuring secrecy and authenticity.
A 'Harry Potter Pikachu fanfic' could be really interesting. It might involve Pikachu somehow getting into the wizarding world of Harry Potter. Maybe it uses its electric powers to help Harry in a duel or it could be a cute mascot at Hogwarts. For example, Pikachu could befriend Hedwig and they could have little adventures together around the school grounds.