A really interesting way would be if there was a prophecy in both worlds about an immortal being who would bring balance. A character from Harry Potter, like Dumbledore who has some form of extended life through his powerful magic, and a character from LOTR, say Gandalf who is a sort of immortal spirit, could be the key players. They could discover that they are both part of this prophecy and have to work together. Their combined wisdom and power would be used to fight against the common threats in both worlds, such as the dark wizards in Harry Potter and the forces of Sauron in LOTR. This would involve a lot of world - building and character development as they bridge the gap between their two very different universes.
In a Harry Potter and LOTR crossover fanfiction, black magic could take on different forms. In the Harry Potter universe, it's often associated with the Dark Arts like the Unforgivable Curses. In LOTR, dark magic is related to Sauron and his minions. A crossover might explore how wizards from Harry Potter's world react to the more ancient and malevolent black magic of Middle - earth.
Well, it could work by integrating the magic system of Harry Potter with the power of Susanoo. For instance, the wizards in the Harry Potter world could learn to harness Susanoo's power through their magic training. They might use spells to activate it, like 'Accio Susanoo' to summon it.
Well, it could start with the Hulk being magically summoned to Hogwarts. He'd be confused by all the magic around him. The Hogwarts professors would be both fascinated and worried about this strange, incredibly strong being. They'd try to communicate with him, but the Hulk might just roar in response. Harry and his friends could be the ones to eventually make a connection with the Hulk, maybe by showing him kindness instead of fear.
It could be a wild adventure where a character from Harry Potter, perhaps using some sort of magic related to immortality, stumbles into the world of LOTR. For example, Hermione could discover a spell that makes her immortal and while exploring the limits of this new state, she somehow gets transported to Middle - earth. There, she might use her knowledge of magic to help the Fellowship in ways they never expected, like using spells to combat the dark forces that the Ring attracts.
In a crossover fanfiction, black magic might be the catalyst for the meeting of the two worlds. Maybe a dark sorcerer in Middle - earth uses black magic to open a portal to the wizarding world of Harry Potter. This would bring about chaos as the characters from both sides have to deal with this new and unfamiliar form of black magic. The wizards from Hogwarts may be shocked at the more primal and ancient black magic they encounter in Middle - earth.
Well, it could work by having a common threat. Say a powerful sorcerer from the Harry Potter world who has learned to harness some sort of high - tech power similar to what the Avengers deal with, and also has a network of assassins at his command. The heroes from each world would then come together. Harry Potter and his friends would bring magic, the Avengers their superpowers and technology, and the Assassin their unique combat and stealth abilities to combat this threat.
A possible way for a crossover to work is through a shared villain. Let's say a powerful dark wizard somehow gets hold of some Stark Industries tech and combines it with dark magic. This new threat catches the attention of both Spider - Man and Harry Potter. Spider - Man would bring his scientific knowledge and superhero skills, while Harry and his friends would contribute their knowledge of magic. They would have to learn to work together, with Spider - Man having to adapt to the strange rules of the wizarding world, like not being able to use his phone or other Muggle technology in Hogwarts.
One more theme is the exploration of different cultures. The wizarding world in Harry Potter has its own unique culture, with things like Quidditch and the Houses. Middle - earth in LOTR has different races like hobbits, elves, and dwarves. In the crossovers, characters get to experience and learn about these different cultures. For instance, Hermione might be fascinated by the elvish language and lore, while Legolas could be intrigued by the spells at Hogwarts.
One idea could be that the One Ring somehow finds its way into the wizarding world. Maybe a Hogwarts student stumbles upon it during a field trip in an ancient forest. Another could be a wizard or witch from Harry Potter's world traveling to Middle - earth to study their different forms of magic. A third idea is that the characters from LOTR end up at Hogwarts to seek help in fighting Sauron, and they learn about magic wands and spells.
It could work by having a magical event that blurs the boundaries between their two worlds. For example, a powerful spell gone wrong could open a portal between the wizarding world and the world of the vampires. Elijah, being curious and always looking for new things, would be drawn to this new world. He and Harry would then meet and their different skills and personalities would create interesting storylines.