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.
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.
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.
Manda might be a long - lost friend of Harry's family. Maybe she was thought to be dead, but Harry discovers a way to summon her through some ancient magic. She could then play a role in revealing secrets about Harry's family past.
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.
Maybe Harry summons Manda because he believes Manda has knowledge about a hidden magical artifact. Manda then guides Harry through a series of challenges to find this artifact which could be the key to saving the wizarding world from a new dark threat.
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.
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.
Yes, it's possible. In many fanfictions, the main character has to find a way to control whatever they summon. Harry, being a powerful wizard in the original series, might use his magic skills and knowledge to control the demon. He could use spells or magical objects to keep the demon in check.
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.