One possible plot could be that Harry uses his magic in ways that are unexpected in the Middle - earth setting of Helm's Deep. Maybe he uses spells like 'Protego Maxima' to create a shield around the fortress, protecting it from the incoming orcs. He could also use the 'Accio' charm to bring weapons to the defenders when they are in need.
In this fanfiction, Harry could have a special connection to an ancient magic in Middle - earth. He discovers that the magic he has from the wizarding world can interact with the magic of Helm's Deep. For example, he could use the magic of the Elder Wand (if he somehow has it) to strengthen the walls of Helm's Deep. His ability to communicate with magical creatures might also come in handy. He could call on some local magical beasts to help in the defense, like getting giant eagles to swoop down on the orcs from above, just like in 'The Lord of the Rings' lore but with a Harry Potter twist.
Well, perhaps Harry is transported to Middle - earth by some magical accident. At Helm's Deep, he realizes the importance of the battle and decides to use his knowledge of magic and strategy. He might teach the defenders some of his Hogwarts - learned dueling techniques to better fight the orcs. And his invisibility cloak could play a role in sneaking around and gathering intelligence on the enemy's movements.
One possible plot could be that Harry and his friends accidentally get transported to the Battle of Helm's Deep. They use their magic to turn the tide of the battle in unexpected ways. For example, Hermione could use a spell to make the walls of Helm's Deep even stronger, while Harry and Ron lead a group of wizards to fight off the orcs from above using broomsticks.
He could use his flying skills on a broomstick to scout the enemy. Just imagine Harry on his Firebolt, flying high above Helm's Deep, seeing the approaching army of orcs and relaying the information back to the defenders.
Obviously, Harry Potter himself would be a main character. He could bring his skills and courage from the wizarding world to the Battle of Helm's Deep. Hermione Granger would also be there. Her vast knowledge of magic could be crucial in finding solutions during the battle. Ron Weasley too, with his loyalty and his own brand of wizarding skills. Maybe Hagrid could also be involved, his huge size and strength being an asset in the physical combat at Helm's Deep.
Well, this fanfiction could start with Harry having a strange vision or finding an old prophecy about the Helm of Fate. He then sets out to find it. Along the way, he could encounter different wizarding cultures and magics that he didn't know before. The Helm of Fate could be protected by powerful spells or guardians. Harry might have to learn new spells or skills in order to overcome these obstacles. Once he finds the Helm, he may discover that it has a connection to his own fate or the fate of the wizarding world as a whole, and he has to make a difficult decision on how to use it.
The Battle of Helm's Deep of the Lord of the Rings was a fictional war in the fantasy novel The Lord of the Rings by the British writer Tolkien. The two parties involved were Saruman's army and the Rohirrim who had taken refuge in Helm's Deep, led by King Theoden. Theoden led his army to reinforce Okenbrand, who was engaged in a fierce battle with Saruman's army. However, Okenbrand's troops had already been scattered before they arrived. Gandalf suggested that Theoden should be stationed at the Hornhorn Fort at Helm's Deep, where many people of the Western Region had fled the war. Helm's Deep was commanded by Gamlin, and the women and children were gathered in Dunharrow by Aeowyn. The elves 'participation in the Battle of Helm's Deep was an adaptation of the movie version. There were no elves participating in the battle in the original novel.
The characters who appeared in the Battle of Helm's Deep were Legolas, Pippin, Meryl, Gandalf, Aragorn, Kinley the Elf, Theoden the King of Rohan, Sauron, Saruman, Aragon, Aeomer, and Haldier.
The plot could start with Harry somehow ending up in Gotham. He notices the Joker's chaos and decides to intervene. Harry might use his invisibility cloak to get close to the Joker without being detected at first. Then he could start using his magic to slowly disrupt the Joker's evil plans.
One possible plot could be that a powerful dark wizard has set his sights on the Cullens for some reason, maybe to use their vampire powers for his own evil deeds. Harry, being the hero he is, senses this danger through his connection to the magical world. He then teams up with the Cullens, teaching them some basic wizarding defenses like Shield Charms. He also uses his own powerful magic, such as the Killing Curse in a defensive way if necessary, to save the Cullens from the clutches of this dark wizard.
Maybe Harry uses his magic skills like the Patronus charm to protect the Jedi from the Sith. His ability to apparate could also help Jedi in quick escapes.
Well, perhaps the vampire is being hunted by some dark wizards. Harry Potter, being the hero he is, uses his magic skills like the Patronus charm to fend off the attackers and save the vampire. Maybe the vampire has some crucial information about Voldemort that Harry needs.