Well, it's quite an adventure. The characters face new powers and limitations as elves. The story might explore how their relationships evolve in this unfamiliar setting and how they handle the unfamiliar magic and creatures.
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.
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.
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.
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.
The Hogwarts faculty would be in chaos. Dumbledore, if still alive in the fanfiction, would be very concerned. He would use his vast knowledge and connections to try and find Harry. Snape might have a more complex reaction. He might initially seem indifferent but secretly start his own investigation. The students at Hogwarts would be both scared and curious. Some might form search parties, while others would spread rumors about what happened to Harry.
For some characters, the Veil of Death in fanfiction can be a source of great fear. They might be haunted by the mystery it holds and what might happen if they get too close. For example, Neville could be depicted as being especially cautious around it, given his past experiences with death and loss in the main series.
Well, it could be a major turning point. It might change the power dynamics in the wizarding world significantly. For example, Voldemort would lose one of his most loyal and powerful followers. This could lead to a shift in the way he operates and make him more desperate or erratic. It also might affect the morale of the Death Eaters, some could lose faith in Voldemort's cause without Bellatrix's unwavering support and leadership among them.
It can have a divisive impact. Some fans might be intrigued by the new ideas presented in the infamous fanfictions and it can spark their creativity. They might start to think about the story in different ways. However, it can also annoy many fans who are purists and want to keep the original vision of the series intact. For example, when a fanfiction changes the essence of a beloved character like Dumbledore into something unrecognizable, it can make traditional fans very unhappy.
It keeps the fandom alive. Fans who read and write this fanfiction are constantly engaged with the Harry Potter world, discussing their favorite stories and ideas. This creates a sense of community within the fandom.