Maybe Firenze the centaur. Centaurs are known for their combat skills and Firenze, being friendly towards Harry, could teach him the art of sword - fighting. He might have a different perspective on combat compared to human wizards.
Neville Longbottom could also be a great sword - fighting instructor in this fanfic. Neville grows into a very capable wizard. He might have discovered an old book on sword - fighting in the Room of Requirement and mastered it, then decided to share his knowledge with Harry.
Well, it could potentially give him an edge in combat. Maybe he can use it to defeat dark creatures more easily. For example, if he faces a Dementor, the sword might be able to disrupt its form in a way normal spells can't.
Dumbledore could also be his mentor. Dumbledore was knowledgeable about many things, and it's possible that he had some understanding of alchemy. He could encourage Harry to explore alchemy as a way to enhance his magical abilities and use it for good.
Dumbledore could be the instructor. He might have a special training program in mind for the students. He could teach them about the deeper mysteries of magic, like the secrets of the Hallows or the true nature of the Deathly Hallows. His wisdom and experience would make him an excellent teacher for the students during the summer training.
Perhaps it could be a long - lost relative of Harry Potter himself. Since Harry has so many mysterious aspects to his own story, it wouldn't be too far - fetched for a relative of his to be the heir. Another possibility is a character from one of the minor wizarding families that has been overlooked in the original series but actually has a strong connection to the swords.
Maybe Naruto is given the dual swords by a mysterious figure. As he starts using them, he realizes that they have a connection to an old prophecy. So he has to master the swords while also dealing with the various factions in the ninja world who either want to help him or stop him because of the prophecy.
Dumbledore could be the one to teach Harry about Haki. Dumbledore, with his vast knowledge of magic and the unknown, might have come across Haki in his travels. He could recognize the signs of Haki in Harry and take it upon himself to guide Harry in understanding and controlling this new power. He would use his calm and wise approach, just like when he teaches Harry about other aspects of magic.
Well, it could be a magical sword like the Sword of Gryffindor. In many fanfics, authors might choose this iconic sword as it has a strong connection to Harry through Gryffindor. It's a powerful weapon against dark magic and has a lot of history within the Harry Potter universe.
One interesting plotline could be that the sword gives Harry Potter some special powers. For example, it might be able to cut through magical barriers that no other weapon can. This would make him more powerful in battles against dark wizards. Another plotline could be that the sword has a mind of its own and guides Harry Potter to hidden secrets or lost magical artifacts. It could lead him on a grand adventure where he discovers things about the wizarding world that were previously unknown.
Well, it could be that Harry Potter uses his magic skills like the Patronus charm against Thor's lightning. Maybe Thor mistakes Harry's magic for some new Asgardian sorcery at first. And then they start to fight over some misunderstanding, like a magical artifact that Thor thinks is from Asgard but Harry knows is important for the wizarding world.
It could be another powerful wizard who wants the grimoire for himself. He might be jealous of Harry's connection to it and will stop at nothing to get it. This wizard could have a dark past and be using the grimoire as a means to gain ultimate power over the wizarding world.