One idea could be a demon from the 'demon' world gets transported to the HP and LOTR universes. It could befriend a Hogwarts student and then together they end up in Middle - earth. Maybe the demon has some unique powers that help in the battles against Sauron. For example, the demon might be able to sense the presence of the One Ring from afar. And the Hogwarts student could use their magic in ways that are new to Middle - earth, like using a levitation charm to move large boulders during a siege.
In terms of the slash relationships, a wizard from HP might fall for an elf or a human from LOTR. Their different magical abilities and cultures would create an interesting dynamic. The demon, if it has a more benevolent nature, could act as a mediator between the different groups. It could use its knowledge of different planes of existence to help the characters understand each other better. For instance, it could explain to the wizards how the magic in Middle - earth is different but still related to their own magic in some ways.
In 'lotr slash fanfiction', a recurring theme is the idea of forbidden love. Given the cultural and social structures within Middle - earth, relationships that deviate from the norm, like a relationship between two male characters from different races or social standings, are often explored. This adds an element of tension and drama. Another common theme is the power of love to overcome differences. Whether it's differences in race, like an Elf and a Dwarf in a relationship, or differences in personality, love is often shown as a force that can bridge these gaps.
One idea could be that the One Ring somehow ends up in the world of Pirates of the Caribbean. Jack Sparrow, being the wily character he is, gets wind of its power and decides to either use it for his own gain or sell it to the highest bidder. The Fellowship, in turn, has to track it down in this strange new world full of pirates, sea monsters, and cursed treasures.
Definitely Legolas and Will Turner. Legolas with his amazing archery skills could teach Will some new techniques, and Will could show Legolas the ropes of being a pirate. They both have a sense of honor and loyalty, so they would make an interesting duo.
One popular 'lotr slash fanfiction' is 'The Ties That Bind' which explores the relationship between Legolas and Gimli in an interesting new way. It delves deep into their friendship and how it could potentially be more. Another one is 'A Different Path' that focuses on Aragorn and Boromir. It's a great read for those who like to see different character dynamics within the LOTR universe.
A lesser - known but excellent piece is 'Ringbearer Buffy'. Here, Buffy is somehow made into a ringbearer. It's a unique take on both franchises. The story delves deep into how Buffy's personality and her experiences as a slayer would affect her role in the quest to destroy the One Ring. It has some great character development and some really intense action scenes.
One thing that can make 'abvh hp slash fanfiction' unique is the way it reimagines the relationships between characters. It takes the familiar Harry Potter world and twists the relationships in new and unexpected ways. This can offer fresh perspectives on the characters and their dynamics.
Another way is to join fanfiction communities or forums dedicated to either Harry Potter or LOTR. Members there might be able to recommend some great slash fanfictions that combine the two. They may also share their own works or works in progress that are of this type.
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.